I designed a relay panel to be hidden under the old battery box. 6 relays - PIAA driving lights, Optional (Stereo Amp), Fuel Pump, High Beams, Low beams, Fog Lamps (PIAA).
Just a note on wiring, I am using the parking/running lights to power the low beams. That way they are on all the time when the switch is pulled. The original mustang wiring turns off the low beams when the highs are on. I will use the low beam circuit to power the fog lamp circuit, and interrupt it with a switch (probably out of the dimmer switch). You usually never use the fog lamps with the high beams on.
