Behind the driver side headlight there's a 1.5" diameter cylinder. Spin off the tethered cap. A piece inside with electrical connectors is free to slide in the cap. Pull it out -- pull it to the end of its travel then rock it out. Carefully scrape off as much of the silicone goop as you can. Don't cut any wires, solder or traces. Use your imagination to figure out what insulating material you can fit between the cap and the sliding piece to put pressure on the solder joints. I used a couple of furniture pads. Some folks use a foam packing peanut or a cigarette butt. Put it back together and tighten it just enough to get the tach to work and maybe a little more. I owe you a worn silver rack damper pin if that doesn't do it.

Oh, also clean the magnetic sensor at the 1 o'clock position over the crank pulley.

Sixto
91 300SE
87 300SDL
83 300SD ... any takers at $1100?