When I added mine, I just "eyeballed" it from the driver's side, centered it in the low spot on top of the fender/door & drilled a couple holes. One this to watch out for is that it should mount a little bit farther back than it looks like it should in order to clear the vent window.

70 Roadrunner convt. street car 440+6, NOS, 4-spd, SS springs '96 Mustang GT convt. street car '04 4.6 SOHC, NOS, auto, lowered "Officer, that button is for short on-ramps"