blower motor resistor
Two that cool the radiator (under the hood) and one that blows out the vents (behind the glovebox)
Check the wires to the blower motor resistor under the passenger side of the dash. The wires will get hot and melt causing an open in the circuit. If this is the case, there is a replacement pigtail and connector assembly available from GM. Also, go ahead and replace the resistor itself because it is probably going to be melted too.
If your blower motor is either not turning itself off, or randomly turning itself on with the ignition off (and you have auto climate control), the problem is the blower control module. Located directly behind the blower motor, it takes information from the climate control, and is in charge of turning the blower on/off. To replace the blower control module, remove the splash shield under the passenger side dash (there's 3 plastic retainers hold it in place). The blower control module is located behind the blower motor, and is held in with 3 bolts. The 2 rearmost bolts (by the firewall) only need to be loosened, and the front one removed. The entire unit rotates down and can then be replaced.
This is most likely a faulty ignition switch. It is a common failure in the early trucks and should solve the majority of your problems listed. www.trailvoy.com Come and look around our enthusiasts forum and see if you can find answers for any other problems you may have!
Defrost is the default position. Most likely a vacuum leak somewhere between the engine compartment and the passenger compartment. Look for a small black hose going through firewall ti the heater box inside. It should have vacuum from the engine on it. if not it's on the engine side. if it does it's probably in the passenger compartment. just trace the line back looking for the leak.
I had this exact same problem.........if it is only overheating while in traffic at an idle and the fans do cycle while the heatac is on then i have good news for you. Between the two fans in the engine compartment, there is a plug that is identical to the ones holding the wire to the fans. Remove this and check to see if it is burnt.. If it is you need a new blower resistor and a new pigtail connector for it.....both are cheap and very easy to replace. peace
You can always replace what you have with newer equipment...I will also offer you the opportunity to possibly re-route some of the mechanical in order to better facilitate any renovations you want to undertake. I'd suggest getting the advise of a professional HVAC person before doing so as you could end up imbalancing the heating/cooling and air exchange in various areas of the house. Having a blazing hot kitchen and a freezing bedroom would be not the best scenario!