1991 Toyota Pickup Electrical

With no details or anything else to go on, the best suspect is the generator. If it has one bad diode of the six, it will only be able to develop one third of its design value for current, and that's not enough to run the entire electrical system under all conditions, especially when using a high-current system like a heater fan motor. The generator should be professionally-tested for maximum output current and this has to be done on the engine. Don't remove it for testing at an auto parts store. The motors in those testers aren't nearly powerful enough to run a generator wide open.