[Lib]

I recall from my old british car days that the main culprit with dim instrument lighting was the rheostat (dimmer) in the light switch. Solution was to jump the hot wire around the dimmer. You could no longer dim the lights, but you had nice bright illumination.

Anyone tried that with the Mustang headlight switch?

[Ivy66GT]

If you rotate the headlight knob all the way CCW I think it shorts out the rheostat to accomplish exactly the same thing without any new wires.

[rocky289]

Not sure about the 66, but if I do that in my 65 it turns on the interior as well.

[alpha male 67]

i used a kit from mustangs unlimited , (white LED's)
here is a write up on it How To Install Led Instrument Lights On Vintage Mustangs - Mustang Monthly Magazine

really like how it looks
here is mine with white face overlays -- ohhh sorry that pic stinks !