The issue with bad grounds is that nothing will make sense. The current will find a way - like if the EDS computer had a bad ground, perhaps it's sending some current through the tachometer now? Or if the instrument panel has a bad ground it would be sending some current back through the EDS? Go into the repair manual and find all the main ground locations and start taking them apart, and cleaning them, one by one. There are a few which will have many wires going to them, I'd hit those first.
I've not worked on a W140; I can't offer any tips on where they might be located
