I am not sure how large or clear the picture below will be, but this is a Challenger wiring diagram I made from the '71 Dodge service manual.
On the left hand side of the pic there is the Panel Dimmer Switch. The tan wire from terminal R goes to a fuse at the fuse block. Turn the switch on and check to see if you have power at the fuse terminal and see if the fuse is blown. From the fuse an orange wire runs to the connector at the radio. This wire feeds the lights at the radio, heater control lamp, and the Orange 3-prong molded plug. If you are getting power to all of these points and the lights still don't work then maybe you don't have a solid ground. If you are not getting power to the fuse your dimmer switch is probably bad. I think the knob has to be turned all the way counter clockwise for the lights to be the brightest.
I am not sure what the red wire is you are talking about. If it is in the bundle of wires that goes to the gauges it is possibly the high beam indicator wire, or if it is a large red wire it might be the wire to one side of the ammeter.
Kyle
