Headlight Issues

Vibration, heat, those wires up front and in the engine bay go through a lot.

Dimmer switches I have never had issues until last week. But its not uncommon for those switches to give out under years of use and being in a location with wet shoes etc dripping on them.

Headlights.
Here's the deal. Yes towrd the end of their lifespans they will be a little dimmer than when brand new.
Yes. The poor ground played a big role in the bulbs producing less light.
If everything was perfect, the voltage across the headlights with engine running would be around 14 to 14.5 V.
With resistance in the dimmer switch, resistance at the poor failing ground, plus the long distance the electrons need to go through 18 and 16 g wire, plus a headlight switch, the votlage across the bulbs might have been 10 V or less.

Your 1971 came with 6012 sealed beams. Sometimes marked with a 2 on the glass.
Nominal rating each 50/40 Watts high/low.
Replacements are generally something that draws more power, but always.
6014 Nominal rating 60/50 watt (Came new on Chrysler products a few years later)
H6014 60/40 W (around 1977) H indicating a halogen
H6024 Since 1984, and typically (but not always) with 35 watt low beam.

Dan Stern has suggested looking for new old replacements if one is sticking with sealed beams. The products currently made are generally produced on very worn molds etc and quality is not as good as stuff made in the 1970s.
If you drive a lot at night on roads that need good light than look into a relays powering the lights more directly from the alternator, and consider the Hella "Vision Plus" US DOT lamps with some good bulbs as a relatively reasonably priced upgrade. A few months ago some guys were able to buy Kioto kits direct from Toyota for a great price for good lamps and relays.

To rough check headlight aiming if you don't have access to an aimer.
https://www.forabodiesonly.com/mopa...dlight-aiming-diagram.509928/#post-1973771432