Well, on a 73 challenger the is 3 positions on the factory dimmer . Power in , power out , courtesy ground.
So for the. New Dimmer you :
hook up the 12v + in. To the in ( my case was black/y)
Hook up the 12v + out to the ( my case brown)
Hook nothing to the ground terminal on new dimmer if your new dash sockets are already grounded .....
Then the factory courtesy. Ground wires( my case the yellows) Both get landed to one side of a new single pole momentary switch , the other side of the new switch gets a new wire to chassis ground .
Long story short.... The two yellow wires are a open ground loop , when grounded it grounds the dome light and turns it on . That circuit contains 3 switches ( left door ,right door, dimmer ) any one of the 3 grounded will turn on the dome .