Confused on wiring. 1964 Barracuda.

What diagram are you referring to and what is the starting points?

I spoke too soon, all I have is 65 which may be different. R6 is the main big red coming in through the bulkhead connector, providing battery power. This goes to the big welded splice, and Q3 feeds from that to the "hot" fuse buss in the fuse panel. So the fuses off that buss feed anything that is always hot, such as the dome circuit, brake lights, etc.

I'm not finding M3 on this one, but if pink it should be the hot feed to the dome circuit. YELLOW is the grounding switch leg that goes to ground through door/ headlight switches.

Find the chart in the manual which lists fuses. That should tell you which fuse the dome circuit was originally fed off of. It may not come direct from the fuse, but may be tapped off elsewhere off a feed coming from one of those fuses.