The box in the back of the bulkhead is for connection of the signals batteries for the radio gear, depending on what it was set up for, clansman or Larkspur systems. I worked with the Larkspur gear myself, but I think a lot of them were changed after I left so not too sure
What does the ammeter read for the vehicle system?
If it had the bit 24v alternator on the engine, try undoing the big plug, and cleaning the multi connector pins
If you can take some pics, of the system it would be a help
Is there a battery box in between the seats?
Worth chescking for corrosion on the terminals, they often got ignored