I recently installed a tachometer in my truck and there's one wire that goes to the dimmer switch. Does anyone know how to access this wire so that the tach is in the same line with dimmer with the other instruments.

 

dimmer is combined with headlight switch, and, if I remember it right, it has 2-wire separate (from lights) connector. One wire is +12V input, other wire is dimmer out.

 

Are you talking about the headlight dimmer or the dash-light rheostat. The dash lights are ran off of a rheostat to dim them, there should be a wire that goes to the heater controls for the lights that you can tap into.