My '91 Dodge Ram Van with over 200K (used to make a Roadtrek Camper) has a unique problem. The backup lights, electric windows, and heater\ac blower are all not functioning and whenever I try to operate one of those systems the brake warning light comes on while say for instance the down window switch is depressed. As soon as I let off the switch, the brake warning light goes out. Btw, the van runs and stops as normal. I have checked fuses and tested for voltage as far as the fuse box in the glove box and have hot wired the blower motor to determine that it does work. Also checked and cleaned all the grounds I could find and determined that the parking brake is not on. I have studied the wiring diagrams and see that the non working systems are all powered through the ignition switch and on the same 12 gauge wire. The break warning light also is powered through the ignition switch and I have looked at the wires as much as I can get to and don't see any obvious connections. Can anyone tell me where I should be looking and how these seemingly unrelated things are affecting each other. Is it the ignition switch? Any help would be much appreciated.

 

Start looking for things they have in common. Since the problem only presents itself when you are actually trying to use them, likely going to be on the ground side of the circuits.

Moving this to the Ram Van section for ya. Welcome to DF!