Electrical 70 Mach 1 interior courtesy lights burn with doors closed

Not everyone is aware that the light switch knob has a detent and when turned CCW past the detent the courtesy lights come on. This could be your problem. Just CW from the detent the dash lighting will be the brightest, and as you continue turning CW the lights get dimmer. When using the dimmer, the light switch needs to be pulled out to either the Park or ON (headlight) position.
Below is a schematic (not a wire diagram) for a 69, which is very similar, but not exactly like a 70. The path marked in yellow shows the knob courtesy light switch:


You can see that the door switches and the light switch are all in parallel, so if any one of them is on, then the interior lights are on.

70ivygreen said:

Something internal to the switch was completing the circuit to the courtesy lights, namely the operating knob. Replaced the light switch, problem was solved. Thank you very much for your assistance.

Click to expand...

Yes, contact D1 to D2 operates the courtesy lights via the knob. I'm glad that helped.
One of the guys on another forum says that it is getting harder and harder to get the correct style light switches and now the only thing available is a "one part fits all" universal type switch that often doesn't fit or solve the problem.