I'd jump fire to the pull apart connector at the dr headlight area (not sure exact loc on '73 RR)/into A bodies only) & see if the brights and dims work (2 wires) which tells you the bulbs/sockets/grounds are good & if good then the next connector upstream probably the bulkhead. If hot there (2 wires 1 for brights 1 for dims) then further upstream to the next connector which'd be the dimmer sw & 1 terminal should be hot with the headlight sw on & 2 terminals 1 to brights 1 to dims. If good there go to the headlight sw.