Voltage at Taillights 66 Dart?

Considering the "not very much current" NORMALLY in that circuit, that is a hell of a lot of voltage drop. LOL. You probably don't even need an infra-red thermometer. Just turn them on, leave them burn for a couple minutes, then feel around the fuse panel, the light switch, and the kick panel connector for heat. And smell LOL

Might also be you have a partial short somewhere drawing too much current. I assume, you did not say, that it is not blowing fuses