My 76 240D has had lots of bad grounds here and there.
The ones in the trunk, under the tail-lights were causing all kinds
of lighting and cluster issues. All I had to do was clean them with
a wire brush and retighten. All fixed.
Also check where the main battery neg cable connects to the body. The sheet metal on 114/115 cars tends to rust there badly.
I added an aux neg cable from my battery directly to the core support just in case.
Dr.D