I have a 1989 190D that has a tach that will work occaisionally. I've replace fuse 10 and that did nothing. The Tach will work for 3 or 4 days, then stop for a few. The only consistent thing is the ABS dash light is on whenever the tach isn't working. When the tach works, the ABS light is off. Any ideas?

Also, is it possible for just part of the combo (Rear defroster, turn signal, wiper) relay to fail? I've been with defrost on the rear for over a year. The grid on the window checks fine, the light on the switch comes on, but I don't hear a click when it happens.

Thanks!