I have 1985 GMC 4/4 it keeps popping fuseable link on fire wall I have checked all the wiring. The red wire that into the fire wall the fuse block show to have direct short when the ignition switch is

At the light switch, red wire is hot all the time. Use a test light to check for voltage. That voltage comes from fusible link at battery junction block. One of the positive battery wires goes to junction block. One way to check fusible link is to pull on it, if it stretches, the link is faulty. There are probably several links at junction block. If voltage on red wire, with headlamp switch on, check voltage on yellow wire with test light, if it's hot, have to check wiring to and from dimmer switch and dimmer switch itself. If red is hot and yellow wire isn't, I suspect the headlamp switch. goodluck.

Did you check for B+ voltage at the alternator with DVOM - digital volt ohm meter ? The heavier gage wire on the back of the alternator should have battery voltage on it. How wires are there in the connector at the alternator ? Does the battery symbol light lit up on the instrument cluster with the key on engine off ? Does the light go off when vehicle is started ?
1992 GMC what ? Look for blown fusible link at battery junction block located on the fire wall near or behind brake booster !

A fuseable link comes through the fire wall. I had to solder a wire to my '92 van to keep it working after it blew. It happened a long time ago and now I'm not sure what it effected.

towing package , the blue wire control the charge coming from the battery sometime could be a big red wire(fusible link or inline fuse near the battery or fire wall

There should be a tag or marking for the amperage on the fuseable link, this should be rated lower [ say the original wire is 8 gage, the fuseable link should be a lighter, like 10 gage ] then the wire it protects and should fail first. Is the spliced wire smaller then the Fuse link?, If so then you should replace the wire with the same size as the orignial one from the factory.
NOTE: Fuseable links have a isulation that's fire resistant, common wire shouldn't be used in this situation as there's a fire danger.

they're speaking of fuseable links. They are larger gauge wires that are in the wiring harness near the battery off the positive battery cable. many times they have "swollen" junctions where they connect to the wiring itself and contain the fuseable element. they carry larger multi-circuit feeds to important components like the fuse box. use a simple light (lamp) driven test light to check them before and after the swollen part of the fuseable link. when you find the bad one replace with an actual fuseable link and not just a big wire because if a short exists (say at the back of the fuse panel) an unfused feed will burn up the panel or even do more damage to the wiring harness, and the cost of the repairs will soar. fire is even a possibility.
NOTE: toyota vans often have very long and expensive battery cables, perhaps even "junctions" where one connects to the next. check these cables while disconnected with an ohm meter or by bypassing them with jumper cables. keep a fire extinguisher handy should something go wrong.

It is a pretty good bet that you fried a number of components, however there is more than one fuse block. Look under the dash. There is also a box that has relays and circuit breakers. Test these too.

check the FUSEABLE LINKS AT THE STARTER
on the big stud you will have about 3 wires they are the( FUSEABLE LINKS) plus the main battery cable
check thoses 3 wires they are most likely the problem
to check them SLIGHTLY PULL ON THE WIRE it will most likely streach and pull apart or there will be BLACK plastic juntions that the wire goes into pull the wire from there
to repair cut the wire pass the BLACK juntions
get NEW FUSEABLE LINK WIRE
USE ONLY HEAT SHRINK **** CONNECTORS tape the wire together
reconnect the 3 wire to the starter
hope this helps

fuseable link? Located on #10 red wire on bat pole of alternator - or is your ground clean and tight located on psgr side - its a braided wire runs from block to frame

Check the fuseable links .. they are a rubbery type wire usually on the back of the alternator or starter motor usually in a group with 3 to 6 of them ..They usually have a black or white piece of plastic where the fuse link attaches to the actual wire .. These wires aka fuseable links usually corrode or rot out at the plastic piece under the insulation you hardly see the break in the wire...good luck..by the way.. these links act as a slow blow fuse in the system...