Audi 80 oil pressure warning system

[a4kcstq at comcast.net]

I can't help you with the wiring, but I think you should start by figuring out why the tach is dead.  The same rpm signal feeds both the tach and the oil pressure controller.  The warning is probably on because you've got no rpm signal.  I'm guessing the oil pressure controller is fine, just missing some input.
If your tach is similar to a 4kq, it will have a +12v wire, a ground, and a signal wire.  You need to find the signal wire and trace it back to the coil or ECU.  I'm not sure where the signal comes from on your car.
-Peter Lines