1980 300sd, honk my horn - killed my tach

[240diowa]

Just bought a 1980 300Sd that has been sitting for 5 years...Nice car, have most things fixed except an electrical issue. My check seat belt light comes on if I do any on the following. turn on or off the heat, use a turn signal, honk the horn, turn/off the radio....basically anything.

Today I honked the horn and the tach stopped working, after I shut the car off and started again the tach was back. I suspect I have a grounding issue. Have found some grounding wires by the radiator drivers side, one on the drivers side inner fender. One for the battery and one from the motor to the chassis. Have cleaned all of those, have installed a new alternator as well.

Question: does anybody know where all the grounding wire to frame locations are?

Don't think I have found them all yet.......