Sounds like the motor is bad at the very least. The resistors will get hot when running the motor. I would expect the lower the speed, (higher resistance) the hotter it would be, which doesn't jibe with what you are seeing.....

Also, the resistor block is in the air box, so that the moving air cools the resistors.

 

That's what I thought too, it didn't make sense that it got hot on medium and not low.

So I dug further into the wire diagrams, and it appears maybe the power from the battery to the rear hvac also goes through a 10a blade fuse in the fuse box to left of the dashboard at some point. So just in case that is relevant, I replaced that fuse too even though it looked ok.

I have now replaced both rear hvac relays, the resistor, and the motor, but they are all loose and not in the housing. The pigtail connector I ordered did not match the melted connector that goes into the resistor, so I ordered what looks like the right one from Rock Auto and it should get here within a week. I am replacing that regardless of what is wrong because it is scary.

Testing this set up so far resulted in SUCCESS!
The blower started right up, and worked at all three speeds! The resistor got hot but did not glow red and no fuses were blown.

​​​​​​​The hard part now will be trying to retrofit the blower into my housing. It came in its own housing but I think I can get it off. Then hopefully it will generate enough air to go through the ducts and not blow a tremendous amount of really old dust all over the interior.

Thanks again so much for your help! I hope this thread helps somebody who runs into the same problem.

 

I would expect the really old dust regardless. Maybe have someone with a vacuum in hand when you have it all assembled, and ready to fire off.

 

So as I said now the challenge is retrofitting the new blower in the old housing. The new blower motor/cage/plug is an exact duplicate, but the old one had a metal bracket build into it that attached to the housing. The new one attaches to a completely different and better housing and the screw holes don't match.

I tried to jerryrig it to the old housing but there is not enough room for it to spin. In fact that may have been the original problem, there is so little room to spare for the original motor to spin, that if the bracket, screws, or housing is the slightest bit off level or bent, the cage binds. Then when you turn it on it blows the blade fuse. I may have to actually cut off the part of the old housing and try to attach the new blower housing in its place.

I have a dremel but I am really worried about cutting something important by accident. I am going to try to make a new bracket but it won't help if the housing is slightly warped or bent

 

So today I spent some time fabricating a new bracket for the blower to retrofit it to the rear hvac housing. I designed it in CAD software for children 😂 and printed it on an old 3D printer that refuses to update or die. It fits the screw holes of the new blower and snaps into a removable piece of the old housing, and I am further securing it with Sugru in case the vibrations could make it come loose. This probably could have been fabricated with some different sized fender washers with a drill and some brazing rods, but I don't have all that so I use what I have.

So after the Sugru cures, I will try to install it in the old housing and hopefully it will work. I am likely going to have to do something similar with the resistor too (but not with 3D filament, something metal of course).

It is so frustrating to have all the individual parts of the rear hvac work now but since they are not exact duplicates of the original system, they take a lot of finessing to put back into the housing. But I post this because I am not the first nor the last Dodge van owner that will have to retrofit parts into an undocumented rear ac conversion, and hopefully it will give some folks ideas on how to make it work again.

 

That was one of my major malfunctions when working on conversion vans. Nothing was standardized, no wiring diagrams. Every van, even from the same conversion company, was different. No list of parts, or sources, so any 'optional' equipment could come from anywhere..... and when things go bad, finding replacements was sometimes extremely difficult, if not impossible.

 

So here is where I am at:

Bracket I made was fine, but the original blower housing was so warped it didn't matter. Since that was permanently affixed to the rear hvac housing, I spent a fair amount of time swearing and destroying dremel blades trying to cut that off, and was mostly successful.

I put the new blower back in the housing it came with, and macguyvered a duct connection with foamboard insulation and flextape into the opening of the hvac housing. It worked, but air was leaking everywhere and not from what I made. The original hvac housing had been cut into in multiple places to accommodate refrigerant line repairs and rerouting over the years, and the connections to the air ducts were toast. It was like sticking fingers into multiple leaks of a dam.

I used up my flex tape, insulation board, and a lot of regular duct tape and patched everything the best I could. Now cold air is blowing through the rear vents for probably the first time in years, and I didn't see any dust! The flow is still pretty weak be due the remaining leaks, where high is really more like medium/low and low is a trickle, but that is fixable in the ling term. Any air is an improvement over what I started with!

Eventually I will probably remove what I did and seal up the hvac housing with spray insulation to see if that is enough to regain sufficient air flow. But I need to run it a while as is and make sure all the problems were in fact solved and nothing fails again. Later on I might get inspired to run new ducting since mine is a straight run up the wall into the overhead cabinets - but that is far down my to do list.

Thanks again for the guidance. I am so happy it finally works!

 

Might consider doing fiberglass for the repair. It's a bit more permanent, and you can make it do pretty much whatever you want. Reasonably easy to work with, just make sure you have good ventilation.