I have a problem with my tach, when I have the car running and turn on the parking lights the needle drops and when I turn on the headlights it goes to zero.

I've checked all the wiring and everything seems to be hooked up in the right order. I also hooked up another power source and nothing changes, even when the dash light for the tach was unhooked and the headlights turned on the needle still drops. The tach wire is hooked to the MSD ignition box like it should be (at least thats what the tech told me).

Any of you guys have a clue as to what's causing this problem.

Rocky