Tach problems rarely involve the tach itself. Behind the driver side headlight there's a cylinder a little bigger than a 35mm film canister. The tach amp is inside that canister. Do a search on methods to get the tach amp working again. Typically what's done is - pry the circuit board from the canister top, stuff a foam peanut or cigarette butt into the cover, refit the circuit board, put the top back on the canister. What the foam peanut or cigarette butt does is press the board pins firmly into the connector. 9 times out of 10 that will get the tach working at least long enough for you to find another foam peanut

Sixto
87 300D