Our 78 had numerous 'wire nuts' used to make
12 volt connections in the o'head wiring - don't know if AS did it or a PO...
Using wire nuts are a big no-no, IMHO, especially for low voltage wiring!
If you happen to be cursed with wire nuts, it's possible one has come loose during you're fan repair...
I think you're only hope is to use a DC volt meter and begin tracing from the Univolt back to the fan POS feed wire...
The POS goes from the Univolt to the
12 volt fuse distribution panel - carefully check each leg of the journey from the Univolt (hopefully the new fuse fixed that problem) to the fuse panel, then on both sides of the fuse that feed the rear fan (and other items)...
An AS wiring diagram of the low voltage system would be helpful...
If you have 12 volts out of the fuse panel on the fan circuit, you'll have to check the overhead wiring -
Assuming that your
12 volt fuse panel is mounted in the front of the trailer - check the front o'head vent ass'y, remove the fixture inside to gain access to the 12 volt wiring - you can usually then see the bundle of wires o'head, as well as the connections to that vent/light and see if you have 12 volts at that location - check the connections carefully, making sure they are clean, bright and tight - used new crimp connectors if required, cutting back the wire ends if necessary to get to some clean, bright wires for a good connection...
If your 12 volt fuse panel is mounted in another location (not at the front), you'll have to find out where the fused circuit wires go from there, to properly check for continuity...
After checking the fwd vent, with still no power at the fan, you'll have to go to the next vent/light assembly and repeat the operation of checking the wires and connections, etc...
You might be able to check the 'color' of the POS feed wire back at the fan and look for that color wire each time you look at the o'head wires - you could use one of those automotive testers that have a sharp needle point to push into the wire to see if it's 'hot' - 12 volt POS - along the way...
Those old fuse panels have a way of hiding bad conducting fuse connections - I'd remove all the fuses, one at a time to check them for continuity, and also make sure that they 'seat' into the connectors without any tarnish to insulate the circuit - you can also use your volt meter on each side of the fuse to make sure the connection is good - I like to remove and re-seat all the fuses when there's a problem...
Hope that helps...