HOW TO MEASURE AND DRILL FOR A 1965 OR 1966 MUSTANG GT FOGLIGHT SWITCH
Using a pencil, draw a vertical line 1 1/8" over from the door post (this will be at the lower, left hand corner of the dash).
Using a straight edge, hold the straight edge at the center of the hole in the door jam where the interior light button switch is located. Draw a horizontal line across the vertical line drawn, making a cross section.
Drill your hole 1/4" above these two crossed lines.
These measurements were taken from a 1965 Mustang Fastback factory GT and will work for a Mustang Coupe, but a Mustang Convertible with a power top may differ slightly.