I was kinda hoping someone else might have had this problem!
The tach in my '86 Jetta has always been a little funky... After startup, the battery light will be on and the tach would either not move, or just sit at ~1000rpm (even with slight fluctuation via the throttle.) Blip the throttle, battery light goes off and the tach works.
The battery light part I get, and the corresponding tach action too. I replaced the alternator (thrashed bearing) and the light/tach stayed the same.
Now all of a sudden it doesn't work at all. There's constant 12v at the W terminal now - and I've never thought to check if there was before (when the tach worked.)