1974 Duster /6 rear lights

Did you reseal your rear tail lights? Those bulbs need ground thru the housings. Easy test is put your running lamps on and use a bare wire from lamp socket to a clean metal body part and see if they work.

I also agree with MoparMat2000 above. That plug can be corroded and no power getting thru. The turn signal switch can also hamper the turn and brake but not the running lamps.

Old cars come with old wiring and dirty connections.
Welcome to the club. Keep us posted.
Joe