The thermostat could be stuck closed. the heater core could also be plugged or the door closes when you use ac and opens when you use the heater may not operate.
air pockets in the cooling system, start by bleeding the lines, plus there is a bypass for the heater core when you have the a/c turned on, or your heater isn't on, this fluctuate between makes and models. but i would go to youtube and look up how to bleed coolant lines, and that should make it easy for you.
I would say that your heater core is ok...it sounds like you have a bled door promlem in the dash.
heater core
You would find the HVAC vacuum line routing in a shop manual for that vehicle.
Would have been built for R12, can be converted to R134a.
Your oil pump is shot.
Sure. You can by-pass the heater core but you won't have any heat for defrosting the windshield should you need it.
no.....the heater would have to be leaking
No because the shape would have two right angles because of the parallel lines and the limited number of angles available...just no.
With the vehicle warmed up and the heater on, grab and feel the two heater hoses. If only one is hot, you've got blockage either in the lines or in the heater core itself. If both are hot, the problem is going to be your blend door.
There are two simple answers 1. the wire has come loose from the horn located near the radiator. 2. the wire has come loose from the horn button on the steering wheel. If these are not the cause than there is likely a broken or corroded wire. The grand wagoneer is notorious for corrosion and i am guessing the wagoneer is the same. good luck.
The light in the switch can not be changed, the whole switch would need to be replaced.