I am guessing it's a similar circuit to my A body.
The dash mounted light has a single wire feeding it.
Thats the power in, the other part of the circuit is
made by the ground on the dash. I'd trace down that
single wire to make sure it goes where it's supposed to.

My car had some really bad amateur wiring done on it. Took a long time to fix. Dome lights came on
with brakes etc. It was a mess. Hang in there and
take it one wire at a time.