i am assuming you mean the switch for changing from one tank to the second tank
if this is correct,either the switch on dash is faulty or the actual changeover valve on chassis rail is faulty
I have a 1996 D R and have the same problem. Apparently the flasher switch needs to be replaced.
heating element stopped working
pressure switch
Heater coil
I would replace the headlight switch as Ford has had their share of these problems.
worn blower motor
If all that is working it might be a short in you wiring. I would recomend checking by the light socket.
bad or dirty float on the fuel pump in the gas tank.
Im not sure if this is your problem but the fuse to switch from awd to fwd is ground not hot. So if the wire is skinned somewhere and grounding out it could cause that.
either the switch is not working or a wire to the switch is broken
Check fuses? Could be a bad headlight switch/ Could be an open in dimmer rheostat?
Blown fuse, bad switch, defective blower motor, or bad HVAC resistor pack. Wet fish caught in fan blades??