I'm helping a friend with his 71 Challenger issue:

Started out with low beam headlights went out - car already had new dimmer switch, and all other lights work; dash, tail, parking.
But when the dimmer switch is activated on high beam, all 4 lights work.

Thought maybe it was the headlight switch, so he bought a new dash switch and I swapped it out for him. The low beams then worked.

Fast forward to last night he was at a cruise night and when he turned on the headlight switch, the same issue, no low beams, so he had to drive home with all 4 lights on high beam.

Before I trek out to his place, looking for suggestions?

Could the low beams on both side be bad? If so, why would they go on when the high beam is activated?
Could the new dimmer switch be bad?

TIA!

With the lights working the power is to the dimmer switch. Start there with a voltmeter and see if the voltage is switching at the dimmer on the wire. Then check it at the bulb. Simple system and should easily be diagnosed in 10 minutes.

Been my experience that low beams do not come on with the high beams. You are see high beams in all four headlights.

Get a meter and troubleshoot the issue. The feed for the high/low low beams comes from the headlight switch to the dimmer switch. If it works in high then the headlight switch is probably not the issue. The dimmer switch then feeds either the low or the high beams. Check that with your meter, at the switch. If you have both there then check for both at the headlights. That will answer your questions.

Throwing parts at it hoping you get lucky is no way to do things.

Bulk head connector ( probably not) but? My guess? Check your grounds

Always start simple.

Pull the headlights and check the bulbs first.

Then check for power at the socket (positive and negative) and then use a ground on the battery and to the socket for power. Power with a battery ground and not grounding through the socket means a bad body ground.

If you don't get power work your way back to the switch. Bulkhead connections, dimmer connections etc.

Thank you everyone -
I brought my FSM and tester to his house and after some tests, we wound up replacing both low beam headlights and that solved it!

Thanks again!

thanks for the report back
Strange that it would lose both low beams at the same time,
Myself, I would still check the grounds and also the charging system to make sure its not overcharging.

It is strange but I have seen it more than once.
I usually chalked it up to people who wait until they can't see anything before they replace the bulbs but high voltage/overcharging could be a culprit.

glad it was a [somewhat] simple fix.
however, as a precaution, check all of your connections on that circuit for corrosion or possibly loose connections to the wires, etc.