1991 Chrysler Fifth Avenue wiring/fuse problem. Help!

It does what if you go up a hill?

A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a 12 volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it's not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.

In this case, since it's a relatively high-current circuit, you may need to use a head light bulb. A brake light will only allow about one amp of current to flow through the circuit. A 9004 or similar head lamp bulb will let at least five amps flow. That will be enough to make the fan motor run, even though it will be at a reduced speed.

That circuit breaker is in the wrong part of the circuit to be of any help in diagnosing the cause of the problem because it is protecting too much stuff. That COULD be a clue, however. There could be broken and frayed wires between the front door hinges that are touching intermittently. If there are, you'll see the test bulb flicker in brightness when you open and close the door.

Do you have automatic temperature control?

Just so I'm clear, the fan stops running. By "heat will turn off", I assume you don't mean the air gets cold or it switches to defrost mode. Switching to defrost when going up a steep hill is very common and has an easy fix.

Since the Air Bag light turns on when the circuit breaker trips, you have another indicator to look at besides the fan. Also, since you have the standard, more reliable, manual heater controls, there isn't likely to be a problem with that system that doesn't take out the resistor assembly. That unit has a built-in thermal fuse that would have blown by now if the fan motor was shorted or dragging. For that reason, I'm leaning more toward shorted wires between the door hinges. Do my light bulb trick but unplug that blower resistor. It's on the top of the firewall, under the hood, on the passenger side.

I bought a '93 Dynasty which is basically the same car, and I ordered it with every option available, except automatic temperature control. I'm smart enough to know how to shove a temperature lever around, and there's very little that can go wrong with that system.