I've been trying to trace a parasitic battery drain in my 88 B250. I used a multilmeter on the battery and pulled each fuse- nothing, Then I started unplugging connectors and found one by the battery that stopped the draw. Wiring diagram said it was for the heated rear window, except I don't have one. I left it unplugged and it turned out the radio and the interior lights in the roof above the engine stopped working.
I got fed up driving with either no radio or a flat battery so I pulled the radio to tidy up the wiring; the door speakers also weren't working. I replaced the factory radio a while back. When I did it, the wiring didn't correspond to the wiring diagram in got from ALLDATADIY so I had to figure it out, end result was a bit of a mess, so first up i tidied things up and replaced the wires to the doors because I suspect they are damaged; the speakers would come on when I went over a bump or around a corner. I tried to pull the speedo so I could check the splices in the harness but I couldn't get the speedo cable out the back, even trying was hurting my arm wedged up behind the dash, alot!
While in there this time I noticed this green connector below was unplugged. It was under the dash behind the radio
again the wiring diagram was no help, it clearly isn't for my car. I decided to plug it back in.
The result, so far, is that that the interior lights come on when the doors are opened, which never happened before and no flat battery. I have no idea what that connector is for or why it was unplugged but so far it seems an easy fix.