1987 diesel fire truck (mini pumper pickup) electrical issue

[Baradium]

I just picked up a 1987 brush truck from the Army for the fire department I volunteer with. This truck has just over 11,000 miles and the base fire dept was hiding it when they got new trucks because they liked it so much (they kept it on the range so it wasn't visible and it'd been left out of inventory for some reason). A senior officer figured it out and made them get rid of it so it went to surplus. It was an in service truck until just before, and was supposed to just have dead batteries from sitting.. I went to get it with that expectation, but I have had no luck getting this truck running.

It turns out the batteries weren't actually dead. In fact, essentially all of the *add on* electrical works. But NOTHING that was factory installed works (also, the add on electrical that was keyed hot doesn't work either).

The only thing that was done to the truck was the radios were removed. I'm thinking that they had the radios hooked into the primary feed for power for the truck and cut that wire, but I can't find it. The truck does have some fairly common emergency services modifications, so while the factory harness itself is intact, the connection for the main power feed wire MAY not be in the same place.

One of the big modifications is a battery master switch, which is done via ground interrupt. The batteries tie together via ground as well as hot, and then the ground wire goes through a switch before going to the engine block. This part of the system is working without issue. Another modification is the main feed battery cable goes to a little distribution block on the firewall (which is where the add on equipment gets its power) before continuing down to the starter. There are a couple wires coming off of this point that could be part of the factory harness, but without having codes to look for I'm not sure. The truck also has an ammeter installed, but it is remote calibrated so it works off of that distribution point. The alternator installation is also different, but I believe it's just because it's a much larger alternator.

I am fairly convinced that the problem is under the dash, but I can't for the life of me find any wires that look to be the cause. Everything under the hood looks to be intact with the modifications. There is a second distribution point that seems to be for a keyed source. I believe the glow plug controller is getting it's signal from this source. With the electrical draw of the glow plugs, I cannot easily just provide power to any of these points for diagnosis. I can disconnect the glow plug controller to try to power the dash, but I'm not sure how much that will help me actually find the problem.

The wiring diagrams I found seem to be gas specific, does anyone have access to a diesel one? This is a military truck, but it was civilian spec, so it is 12volt.