Why the 30 amp fuse under hood for heater fan keeps blowing

Check the 50 amp heater fuse under the hood, if it's ok go directly to the fan under the glove box and remove the connector from it, test the motor by putting a hot jumper wire to one of the motor terminals and ground the other side with another jumper, if the motor runs in either direction replace it.

What all does that fused supply B+ voltage too ? Do you know ? That would be your first thing to find out ! Do you know what a electrical distribution diagram is ? Do you know how to test electrical circuits with a DVOM - digital volt ohm meter ? Do you know what a short to ground is ?
Which fuse ? IGN A - 40 amp ,under hood fuse / relay box ? This fuse powers the Starter Relay and Ignition and Start Switch
Does your vehicle have a manual or automatic transmission ? The starter relay could be shorted inside , It's also located in the under hood fuse box . If you pull the relay out of the box it's plugged into an look on the bottom of it you would see four sets of numbers ! 30 & 87 an 85 & 86 . If you were to jump 30 to 87 in the relay box the starter should spin the engine over , not start an run just spin ! 85 & 86 is the control side . Could be the ignition switch !
That 40amp fuse powers a lot of subsystems !

Here is a web site for free diagrams !
http://www.bbbind.com/free_tsb.html Enter vehicle info. year , make , model an engine , Under system click on electrical distribution , under sub system click on power distribution . Then click the search button .Click blue links one at a time . First blue link scroll down to the second diagram . You will see Ign A 40 amp fuse at the top of the page .you will also see it powers the igintion switch ,look at last part of ignition switch , when in start or run that 40 amp fuse powers all those other fuse's plus three relays i see there also . A/C compressor relay , rear window defroster relay ,daytime running lights . Any one of those circuits could have your short . You need to isolate the circuits , pull fuse's till the fuse doesn't blow !

Check the 30 amp fuse? don-ohio (:^)

Well,you eliminated the resistor,so it probably means the blower motor has shorted out.Either that or something has gotten into the blower wheel and jammed it. If you can bypass the resistor and just put 12 volts into the motor,do it.But don't hold it long,as a direct short can cause a fire without a fuse to blow for safety.
In other words,prepare to buy a new heater blower motor. don-ohio

You have a bare wire somewhere either in your instrument cluster or under the hood where the blower motor is. It's located on the passenger side behind your computer it's easy to inspect the wires there cause they are all exposed.If a bare wire is showing just tape up with electrical tape or duct tape to keep it from arking out. If not that check that aground wire isn't fastened somewhere under the hood or on firewall. Check wires from fuse box all the way through the firewall could be dangerous. Also make sure you have the right amp fuse in if it tells you to use a 15 or 20 or 30 amp fuse make sure you have that one in.A lower number fuse will heat up and blow everytime.

IF YTOUR TALKING THE FAN CHECK HEATER FAN FUSE IN THE FUSE BOX OTHER THAN THAT YOU HAVE TO ADD MORE INFO TO YOUR QUESTION LIKE ITS THE FAN WON'T BLOW OR WHATS IS NOT DOING IF ITS NOT THAT FUSE THEN WE WILL HAVE TO CHECK THE RELAY OK POHL CAT

It is the fuel heater, All you have to do is unplug it.

your blower motor is dead replace it.it located either under the hood in the engine compartment on the fire wall or under the dash on the passengers side.It is not hard to do if you are mecanicaly inclined.cost between $30/$100,if you do it.

There are two fuses for the fan. One is probably a 20 Amp in the fuse panel in the cab and marked "Heater". or "Heater/AC" The second is under the hood may be marked Heater, Blower, or Fan and should be 30 Amp. The one in the cab supplies switched 12 volts to the fan speed switch. The one under the hood supplies the 12 volts to the relay. The relay is normally energized by the 12 volts through the 20 Amp fuse in the cab and through the switch. I think you find that the fan has the normal speeds except for missing HI with the underhood fuse removed, and will turn off with the ignition that way. There is no hazard in operating it that way but you may want to replace the resistor/relay assembly before it gets too cold. The relay is a pretty trouble free device in most cases, maybe you can find a deal on a good one at a junkyard.

The whole idea with the relay under the hood started back in the Sixties believe it or not. There is considerable voltage drop in the wiring between the fan switch and the blower fan motor. By adding the relay under the hood right next to the motor, the voltage drop was reduced, both by the shorter wire, and by using heavier wire to boot. The motor gets really close to the full system voltage when the relay is closed. In your case where the relay keeps the fan running, it can drain a battery rather quickly too!

There is another fuse box on the drivers side of the dash that can not be opened unless you open the drivers door because it is located on the end of the dash. The legend for which fuse is for what purpose is on the lid that comes off when you turn the lock mechanism counter clock wise. I found this while searching for the fuse for the 4wd