My 85 fox has an aftermarket sports comp tachometer, it is wired to the fuse box, it works fine when I'm driving during the day, but when I turn my headlights on it shorts out and does not work, and suggestions on what it could be? Thanks.
check the wires for the bulb to the tach. My guess is you are sharing the ground for the tach and the bulb and the + wire for the bulb is touching ground. You do have an inline/ add a fuse for the tach right? Also the ground wire should be to the chassis or block.