69 Dart Headlights Issue

Here is how this works and the functional path

FIRST there IS NO fuse for the headlights. The power ONLY for the lights (not park/ tail, etc) comes off the black ammeter wire, the "welded splice"

There's a breaker right in/ on the light switch for that circuit.

With the switch "clear out" headlights power is fed TO the floor dimmer switch. Two of the terminals should be hot with the light switch on.

The dimmer is a simple SPDT (2 way, or double throw) switch. Power from the light switch feeds to the dimmer, and when you punch it, it switches the output from one terminal (hi) to the other (lo.)

THEN the lo and the hi beam power feeds through the bulkhead connector, ANOTHER trouble spot

When the hi and lo beam wires reach the headlights, both of them are grounded through ONE wire at a grounding point on the rad cross support. Check that.

In rare cases where EG the VR is over voltage, etc, all the lights can burn out

HOW TO CHECK

One easy place to start is the dimmer switch. Pull it out or probe the connector in place, and with the light switch, "on" you should have power at two of the three terminals. If only one is hot, likely a bad dimmer. Does the hi beam dash indicator light? Then it's likely either bulkhead connector, damaged harness in the engine bay, bad headlights, or that ground I mentioned.