I totally agree with Reed, your description doesn't exactly match what I'd expect on a 78 & up. Please post a couple of pictures. I'll give it a shot till then.
At the top of this picture are the factory AC/Heater and fresh air/recirculate/blower systems from the later vintage vans:
Do you have a voltmeter/ohmmeter? You should definitely be able to bring her back to life.
First off, based on your description, there is one possible way to cut down on at least some of the hot air you're feeling if your heater box has the normal type of control lever. Connect that cable! When you connect the cable to the lever at the top of the box, your hot/cold lever on the dash will actually be doing something useful. Setting it to cool should block off the hot air leakage from that system.
They used an odd little self-adjusting spring clip to couple the bicycle cable to that lever. Is your clip deformed or missing? I've also seen that lever rust up inside the box at its pivots so that it wouldn't move... it's supposed to move pretty freely. If your lever doesn't move freely, that is likely the culprit behind your issues with that cable. Disconnect the cable, then try applying some penetrating oil to the shaft and work it back and forth a bunch of times, while adding more penetrating oil, to see if it will free up. There's only the shaft and the plastic lid - no real bearings. If not able to move freely, it could be the pivot at the bottom of the shaft, which is like a little molded cup in the bottom of the heater box. You'd have to open up the box (-no fun!) and cleanup and burnish the pivot surfaces of the lever shaft. I'd use grease as a lubricant/corrosion preventative. They should have made them from stainless steel...
A fan not working can come from a number of failures; the switches often fail internally. They're still commonly available at auto parts stores. The wiring can also overheat and the slide-on connector terminals can burn up - if you see discoloration of the wire insulation or metal terminal lugs, this has been going on. The terminals inside the big bulkhead connector can also corrode and fail.
When you said it only worked intermittently on high speed, that would be a typical failure of the switch, or possibly of the blower resistor assembly, which is located on the air plenum behind the plastic tray insert in your glovebox. The switches can look OK but be faulty, and the resistors can fail, both electrically and from other issues, such as corrosion due to a leaking windshield frame:
Blowers draw quite a lot of current; a minimum of in the 15 ampere range on high speed. They usually fused these with a 20 amp fuse all by itself; nothing else sharing the circuit. Later vans used 30 amp fuses on this circuit.
The blower motors can also fail, as can their bearings, and wiring. Blower motors can still be had in parts stores if necessary, as can the squirrel cage fan. They run on 12 volts; you could jump the battery voltage directly to it temporarily and see if it spins up. The ground wire for them fed through the firewall and bolted on the interior by the engine cover, in front of the passenger seat. In addition to making sure the motor bearings aren't seized up, it might be worth a look to be sure the ground wire is still connected.
A factory service manual (FSM)is a critical piece of life support for these old girls, so please keep looking! You may find one occasionally on eBay, or from other sources. I got a digitized version of mine on compact disk from
www.moparmanuals.com - not cheap, but mighty handy and easily tranportable.
I do have a 77 FSM and an 81 FSM, which bridge the two vintages of van Reed and I figure you might have.
Here is the section from the 77 FSM which shows the fuse block, blower switch, resistor, and the blower motor.
Here is the complete isolated circuit diagram of the heater blower system for a '77.
This is the fuseblock description from an 81. I don't seem to have the very same page for the 77.
The verbal description of which fuse feeds which circuit was molded onto a plastic cover which snapped over the fuseblock. It typically split apart or got set aside and misplaced...
Good luck!