I'm trying to get my '73 Road Runner rust-bucket back on the road. It's been down since around 1994. I fixed the oil pan, brake lines, floor pans, and was ready for a Friday night test run...almost. Now there's no headlights! Two trips to the auto parts store later and I have a new dimmer switch and new headlight switch, but still no headlights. I now have marker lights, but no headlights(yes, I did pull the knob out all the way! ). Am I missing something here? Inline fuse or something that would affect headlights but not affect marker lights, or is the only possibility a faulty plug at the headlight switch or at the dimmer switch?

no fuse for headlights- built-in circuit breaker in headlight switch, check at bulkhead connection for melted/corrosion wires, test with meter at bulbs for power.

was the connector for the headlight switch melted???? maybe loose there.

Do the dash lights or parking lights work?

The headlights ground right behind the battery. It is not uncommon for that wire to break or corrode.

Check for Positive voltage at the headlight connector using the battery as the ground connection.

If you have voltage check the ground. If not check the bulkhead and then the dimmer switch on the floor pan. Very common to corrode or melt.

Also check the back of the switch for voltage.

Parking lights and dash lights work. The plugs for the headlight switch and dimmer switch both appear ok, no meltdown (I did have a headlight switch plug meltdown in my '73 Charger once though, so I'm familiar with what that looks like).

Would a bad ground issue at the battery affect the headlights without affecting the parking lights, or would that problem affect both the same?

Possibly because there is another ground in the trunk that may feedback to the parking lights up front as well as I think there may actually be 2 ground wires up front.

Check for voltage at the headlamp and then check for continuity to ground at the headlamp.

If those don't work check for voltage at the dimmer. If you have voltage at the dimmer but not out front it is probably the bulkhead.

If you do not have voltage at the dimmer check the wires from the dimmer to the switch.

Headlights should be Light Green on the switch.

Is the dimmer switch grounded? If I remember correctly, it needs to be grounded. You said you replaced the floors, is the switch even bolted down at this point?

Dimmer switch does not need to be grounded, parking lights & tail lights ground through their mounting bolts, headlights need separate ground wires which will be bolted to the radiator support typically near the battery ...

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.