what year is the challenger? does it have standard dash or rallye dash?
would expect there to be a rotary panel dimmer switch somewhere. either seperate (toggle switch for lamps, seperate roller switch for dimmer B and E bodies i think) or as part of a pull and turn headlmap switch (these might be A body only)
i'd take it out dimmer, start car, then use meter, to find the 12v feed in you will need lights on.
one of the wires will be and should show 12 volts when lights on and 0v when lights off (connect meter between the conector and clean metal on the body/dash)
that proves power to this dimmer switch
(if no power you have a fuse or fuse box issue or the wire is disconnected at the light switch)
if 12 volts found with igntion and lights on,next we need to work out if the dimmer works
mark the feed in and check all other contacts
note any with 12 volts
roll rotary switch back and forth you are looking for a contact that varies voltage with position of switch 0 -12 volts
you may find 2....!
one is for the dome light control and will allow you to dim the dome light or switch it on fully i think only when the doors are shut, i think the dome comes on full power when igntion off lights off and doors open..... can't remeber
one feed is for the dash illumination light bulbs
disconnecting them both and reconnectibng back noteing what goes off will help identify
you want the one for the dash lights
id then use one of these
https://uk.farnell.com/molex/19013-0029 ... price=true
crimp a new wire of long length (red or ornage in your case i think for dash lights)
put this new connector onto the dimmer switch and connect the orginal wire to the piggy back spade
you now have a dimmer controlled feed for any extra gauges lights and its a nice long new bit of wire that you can route over the steering column and along the loom in the bottom of the dash metal work to your gauges by the radio .
If you do not have a dimmer switch for the dash lights..then
you do the same process but this time you are looking for a orange or red off the light switch
it would most likley be the feed to the dash lamps
i.e does it only have 12 volts on it when the lights are on....side, dipped head, or main beam? if so again the piggy back spade and female spade connection can be used.
keep bulbs in gauge lamps to only 12 volt/ 5 watt and you should not have an issue
you need to connect in parrallel so from each guage bulb connect 1 wire to you new feed wire
and either the body of the gauge (metal push fit bulb holders) or if you have 2 wire bulb holders (usually plastic with no metal contact on the push fit surround) the other wire is connected to the car body
if you connect in serial i.e + wire for guage 1 to guage 2 guage 2 to guage 3 and the guage 3's + wire to the new wire the 3 gauge lights will glow at 1/3rd brightness as each will be operating at only 4 Volts. 3 gauge bulbs x 4 volts= 12 volts in total.
you want all 3 to see 12 volts so they each need a wire going to your new feed wire, which brings 12 volts from the dimmer or light switch.
in the picture below the red connector in each is 12 volts and the black one is the car body
left picture the end of each bulb on the red connector side see's 12volts
on the right picture all 3 have 12 volts spread eaqually across them and as the bulbs are all the same its 4 volts across each hence 1/3rd brightness
Dave
No separate dimmer, but the lights are on a flick switch, so never noticed a rotating option. 
