Once removed from the dash cluster, dash-lite dimmer control is EZ to troubleshoot.
The majority of problems are due to the rheostadt windings being dirty, after 50 years..
Clean with some spray contact cleaner like Deoxit, let it dry and then very lightly coat the exposed wire windings with dia-lectric grease. Also now is a good time to make sure the contacts have enough pressure on the windings.
Since the OE dash lites are dim to begin with, one thing I have done on many Mopars is to put a short jumper wire between the rhoestadts' contacts so basically it is bypassed and the full 12V goes to the dash lites.

Just my $0.02...