ok... here is how it goes on 3rd Gens chargers and I'm pretty sure E bodies are the same or similar

input to the dimmer switch comes from the parking light circuit built inside the switch, leaves the headlight switch ( rheostat wheel ) in a tan wire up to fuse box and gets one fuse isolated from the BATT and ACC buss bars, just right a side of the blue wired fuse which is for the ign1 circuit, such as cluster brake light, seatbelt relays if equipped and in some cases, cluster voltage limiters, some other RUN circuit accesories.

once reach this fuse, leaves the fuse box in a orange wire and this is the wire which is spreaded up everywhere ( cluster, shifter lettering, radio faceplate, cigar lighter light, acc switches, AC/heater control switch face ).

What you can make to check ?

feed the tan wire side of the fuse with a jumper wire from the batt side buss bar. Lights should come on full. This will mean the fuse and the orange circuit is OK and the source is not sourcing. This could meant the switch is damaged OR the rheostat dimmer wheel. Headlight switch is not feeding the rheostat circuit from inside OR point/ring what feeds the rheostat is dirty or rheostat itself is damaged, sulfated.

another point to check... are you getting parking lights ? if not, time to check also this fuse. As mentioned the dimmer circuit is feeded from this same circuit

This is a checking for integrated dimmer wheel on headlight switch, such as A bodies, 71 and lates B bodies. Of course Rallye E body is not like that BUT I guess must be kinda similar on how they are wired and work. Just the dimmer wheel is not part of the switch assembly, but must be feeded the same from headlight switch with a wire from it