Our 19yr old has a W123 1983 300d-turbo.
We've had the rear axle assembly replaced, rear breaks serviced, front CV's replaced.
Cooling system just flushed and filled.
We're told the A/C appears sound but "someone messed with the wiring" - any chance you have a wiring diagram and/or some pictures that could help us with that?
Also, it looks like you've made some performance mods to your 1982, why relocate the battery to the trunk? (At 12vdc high-current draw loss is high over even short distances.)
Thanks - David

 

Here's the wiring info:

Compressor Relay in 83 300D Turbo??? - Mercedes-Benz Forum