Re-flow the solder joints on the tach amp. Thats the most common issue. For my year, its a round black piece on the driver's side inner fender. Lots of info on how to do it and its pretty easy. Fixed mine in ~30 minutes.

I didn't add any solder. Just used my iron to melt and re-flow each solder joint on the back side of the unit. It's covered in glue so take your time to clean it off.

https://www.benzworld.org/threads/the-permanent-tachometer-amplifier-fix.1652194/