Ignition switch fusible link blows after starting and running for a short time

The power goes from the battery to the fuse box and then to the fuel pump relay, so just concentrate on the fuse box. Depends on vehicle and model whether fuse gets power anytime the battery is connected (hot at all times) or only when the key switch is in "on" (hot in run or start). You have a test light to check for power at the fuse terminals? That is how to start.
Some older models used "fusible links" in major circuits to distribute power, like to the fuse box, or to the ignition switch. The links act like fuses to protect wiring, they will "blow" or burn out if a big short or surge tries to damage wiring. Then you can just cut out and replace the fusible link. Nowadays, they mostly use maxi-fuses instead of fusible links, but you would need to consult a wiring diagram for your vehicle to really know the set up.

If the truck had a three terminal one on it then you would not use the (I) terminal, nor would it cause you to keep blowing your fusible link. It would cause a hard start cold because all the (I) terminal does is feed 12v directly to the coil on cranking. You have a short in the circuit that is on that fusible link. You need to get a wiring diagram and see what that circuit feeds. If the fuel pump isn't running than disconnect that circuit and see if it keeps the link from blowing. You may have to get a circuit breaker to install temporarily it to help find short. I have used a breaker and have gone directly to the fuse box and feed there to help isolate which circuit it is in. Good luck.

Without knowing the car, the best place to start is at the fuse box. Identify the fuse that feeds the switch. Assuming it's good, check power to the fuse. If it's bad there, check for a fusible link between the battery and fuse box. If you have power to the fuse and the fuse is good, start checking your wires. If the fuse was blown, replace it. If it blows again, you likely have a short that could be anywhere up to the switch and anything fed by the switch.

A bad short blowing the fuse. Could be in the wiring for that circuit, or bad part, component (like the computer, sometimes a shorted injector).
Check if the fuse still has power to it. Use a test light to check for power at the fuse. If no power, probably blew a fusible link or maxi fuse that is the feed for that fuse block.
Turn key to on-do the warning lights and dash gauges come on? If not, check that power is still getting to the ignition switch. I think there are fuses (AM1 &2?) for the ignition switch, not fusible links.
Shorts in the wiring can be difficult to find, may be buried deep in a wiring harness, one needs to check continuity of wiring to locate wiring shorts, looking for high resistance in a section of wire-you may need a shop to diagnose. If you have a wiring diagram available, could start checking wiring and parts in that circuit.
If you want to get creative, here is how an auto electrician can hunt for shorts: use some jumper wires and hook up a circuit breaker of same rating as the efi fuse in that fuse slot. Now when testing you will not be constantly blowing the fuse-circuit breaker will reset every time. Unhook parts in the circuit one at a time, turn key on so power is going through the circuit, whatever part is unplugged that breaker does not trip, that is the problem-the wiring to it or the part itself. Also do a wiggle test of the wiring: key on, move different parts of the wiring and harness, if you cause the short tripping the breaker (or blowing the fuse if you just keep adding a new fuse each time) then you have found the short.
Here's a heads up: anytime you want to pull the connectors off of the computer, make sure key is off, remove negative battery terminal, pull connectors off the computer, reattach the negative battery terminal. With connectors off examine connector pins for any corrosion or bent pins.
Good luck, get power back to the ignition switch if it is lost, then start looking for what is blowing the efi fuse.

most likely for the ignition circuit. a new link must be soldered into place.determine the cause of the short or new link will blow as well.

1 Inspect Battery Damaged or faulty battery and/or connections. 2 Inspect Coil - Ignition Damaged or faulty ignition coil or ignition coil circuit. 3 Inspect Fuel Filter Clogged or dirty fuel filter 4 Inspect Fuel Injector Pressure Regulator Faulty fuel injector pressure regulator or circuit. 5 Inspect Fuel Pump Improperly functioning fuel pump or circuits. 6 Inspect Wireset Damaged, worn, or deteriorating spark plug or coil wire(s). 7 Inspect Control Module-Ignition Faulty ignition control module or circuit. 8 Inspect Solenoid Switch Damaged or faulty solenoid switch or solenoid switch circuit. 9 Inspect Starter Damaged, worn, defective, or faulty starter or starter related circuit. 10 Inspect Neutral Safety Switch Damaged or faulty neutral safety swittch or circuit. 11 Inspect Fuse Corroded, burned, or shorted fuse may be interupting power source. 12 Inspect Fusible Link Corroded, burned, or shorted fusible link may be interupting power source. 13 Inspect Battery Cable Corroded, broken or poorly connected battery cable. 14 Inspect Spark Plug Wire Loom - Performance Improperly adjusted or worn/malfunctioning carburetor. 15 Inspect Distributor Cap Loose or cracked distributor cap 16 Inspect Distributor Cap And Rotor Kit Loose, cracked, or worn distributor cap and ignition rotor. 17 Inspect Fuel System Pressure Incorrect fuel pressure being delivered to carburetor or fuel injection system. 18 Inspect Fuel Injector Plugged, dirty or poorly connected fuel injector. 19 Inspect Ignition Switch Worn, damaged or faulty ignition switch or circuit. 20 Inspect Spark Plug Fouled, damaged or broken spark plug(s). 21 Inspect Belt Incorrectly routed, adjusted(tensioned), missing or worn drive belt(s). 22 Inspect Timing Belt Slipped or broken timing belt. 23 Inspect Timing Set Slipped timing chain or worn timing gear(s). 24 Inspect Fuel Tank Contaminated fuel from rusted or deteriorated fuel tank. 25 Inspect Ignition Timing Incorrectly adjusted engine timing.

1 Inspect Battery Damaged or faulty battery and/or connections. 2 Inspect Coil - Ignition Damaged or faulty ignition coil or ignition coil circuit. 3 Inspect Fuel Filter Clogged or dirty fuel filter 4 Inspect Fuel Injector Pressure Regulator Faulty fuel injector pressure regulator or circuit. 5 Inspect Fuel Pump Improperly functioning fuel pump or circuits. 6 Inspect Wireset Damaged, worn, or deteriorating spark plug or coil wire(s). 7 Inspect Control Module-Ignition Faulty ignition control module or circuit. 8 Inspect Solenoid Switch Damaged or faulty solenoid switch or solenoid switch circuit. 9 Inspect Starter Damaged, worn, defective, or faulty starter or starter related circuit. 10 Inspect Neutral Safety Switch Damaged or faulty neutral safety swittch or circuit. 11 Inspect Fuse Corroded, burned, or shorted fuse may be interupting power source. 12 Inspect Fusible Link Corroded, burned, or shorted fusible link may be interupting power source. 13 Inspect Battery Cable Corroded, broken or poorly connected battery cable. 14 Inspect Spark Plug Wire Loom - Performance Improperly adjusted or worn/malfunctioning carburetor. 15 Inspect Distributor Cap Loose or cracked distributor cap 16 Inspect Distributor Cap And Rotor Kit Loose, cracked, or worn distributor cap and ignition rotor. 17 Inspect Fuel System Pressure Incorrect fuel pressure being delivered to carburetor or fuel injection system. 18 Inspect Fuel Injector Plugged, dirty or poorly connected fuel injector. 19 Inspect Ignition Switch Worn, damaged or faulty ignition switch or circuit. 20 Inspect Spark Plug Fouled, damaged or broken spark plug(s). 21 Inspect Belt Incorrectly routed, adjusted(tensioned), missing or worn drive belt(s). 22 Inspect Timing Belt Slipped or broken timing belt. 23 Inspect Timing Set Slipped timing chain or worn timing gear(s). 24 Inspect Fuel Tank Contaminated fuel from rusted or deteriorated fuel tank. 25 Inspect Ignition Timing Incorrectly adjusted engine timing.

sounds like a bad alternator try this list too 1 Inspect Battery Damaged or faulty battery and/or connections. 2 Inspect Coil - Ignition Damaged or faulty ignition coil or ignition coil circuit. 3 Inspect Fuel Filter Clogged or dirty fuel filter 4 Inspect Fuel Injector Pressure Regulator Faulty fuel injector pressure regulator or circuit. 5 Inspect Fuel Pump Improperly functioning fuel pump or circuits. 6 Inspect Wireset Damaged, worn, or deteriorating spark plug or coil wire(s). 7 Inspect Control Module-Ignition Faulty ignition control module or circuit. 8 Inspect Solenoid Switch Damaged or faulty solenoid switch or solenoid switch circuit. 9 Inspect Starter Damaged, worn, defective, or faulty starter or starter related circuit. 10 Inspect Neutral Safety Switch Damaged or faulty neutral safety swittch or circuit. 11 Inspect Fuse Corroded, burned, or shorted fuse may be interupting power source. 12 Inspect Fusible Link Corroded, burned, or shorted fusible link may be interupting power source. 13 Inspect Battery Cable Corroded, broken or poorly connected battery cable. 14 Inspect Spark Plug Wire Loom - Performance Improperly adjusted or worn/malfunctioning carburetor. 15 Inspect Distributor Cap Loose or cracked distributor cap 16 Inspect Distributor Cap And Rotor Kit Loose, cracked, or worn distributor cap and ignition rotor. 17 Inspect Fuel System Pressure Incorrect fuel pressure being delivered to carburetor or fuel injection system. 18 Inspect Fuel Injector Plugged, dirty or poorly connected fuel injector. 19 Inspect Ignition Switch Worn, damaged or faulty ignition switch or circuit. 20 Inspect Spark Plug Fouled, damaged or broken spark plug(s). 21 Inspect Belt Incorrectly routed, adjusted(tensioned), missing or worn drive belt(s). 22 Inspect Timing Belt Slipped or broken timing belt. 23 Inspect Timing Set Slipped timing chain or worn timing gear(s). 24 Inspect Fuel Tank Contaminated fuel from rusted or deteriorated fuel tank. 25 Inspect Ignition Timing Incorrectly adjusted engine timing.

1 Inspect Battery Damaged or faulty battery and/or connections. 2 Inspect Coil - Ignition Damaged or faulty ignition coil or ignition coil circuit. 3 Inspect Fuel Filter Clogged or dirty fuel filter 4 Inspect Fuel Injector Pressure Regulator Faulty fuel injector pressure regulator or circuit. 5 Inspect Fuel Pump Improperly functioning fuel pump or circuits. 6 Inspect Wireset Damaged, worn, or deteriorating spark plug or coil wire(s). 7 Inspect Control Module-Ignition Faulty ignition control module or circuit. 8 Inspect Solenoid Switch Damaged or faulty solenoid switch or solenoid switch circuit. 9 Inspect Starter Damaged, worn, defective, or faulty starter or starter related circuit. 10 Inspect Neutral Safety Switch Damaged or faulty neutral safety swittch or circuit. 11 Inspect Fuse Corroded, burned, or shorted fuse may be interupting power source. 12 Inspect Fusible Link Corroded, burned, or shorted fusible link may be interupting power source. 13 Inspect Battery Cable Corroded, broken or poorly connected battery cable. 14 Inspect Spark Plug Wire Loom - Performance Improperly adjusted or worn/malfunctioning carburetor. 15 Inspect Distributor Cap Loose or cracked distributor cap 16 Inspect Distributor Cap And Rotor Kit Loose, cracked, or worn distributor cap and ignition rotor. 17 Inspect Fuel System Pressure Incorrect fuel pressure being delivered to carburetor or fuel injection system. 18 Inspect Fuel Injector Plugged, dirty or poorly connected fuel injector. 19 Inspect Ignition Switch Worn, damaged or faulty ignition switch or circuit. 20 Inspect Spark Plug Fouled, damaged or broken spark plug(s). 21 Inspect Belt Incorrectly routed, adjusted(tensioned), missing or worn drive belt(s). 22 Inspect Timing Belt Slipped or broken timing belt. 23 Inspect Timing Set Slipped timing chain or worn timing gear(s). 24 Inspect Fuel Tank Contaminated fuel from rusted or deteriorated fuel tank. 25 Inspect Ignition Timing Incorrectly adjusted engine timing.