Voltage at Taillights 66 Dart?

Got a point and shoot laser thermometer? To drop 9V, something will need to dissipate 3V of power and that can only be heat. you can shoot he suspect joints and switches to find a raised temperature. Disconnect the power feed to that circuit and remove the bulb and check OHMs on that line to ground, it should be OL (open loop) or 999Mohm. That is reading potential to ground. Any reading under 1Mohm would be suspect.