What if your head lights quit working but your parking lights work on your GMC truck?

From what I understand of your problem, IF the circuit breaker [or fuse] for that circuit IS good, then the problem is probably that the wiring of that circuit has what is called an "open circuit condition. i.e., is disconnected somewhere.

For an electrical circuit to function, the circuit must be "complete" which means that a conductor [wire] must be attached to the Positive terminal of the battery and run [through various harnesses and connectors, plugs, etc.] to the fuse, then to a control switch ], then to the electrical device (in this case your headlights), and then another wire [called a ground] back to the Negative terminal of the battery. Usually, the ground wire does not go all the way back to the battery Negative terminal, but instead connects to the body sheet metal near the device, and the body sheet metal is all grounded to the Negative terminal of the battery.

This continuous line of electrical conduction makes a "circuit." IF any part of that circuit is disconnected [for example, at a connector, or a screw terminal, or at a sheet metal ground screw], the circuit is said to be "open," incomplete, and cannot conduct the electricity.

The "fix" for your problem is for someone who understands and is experienced in automotive electrical systems to fully "troubleshoot" that electrical circuit to find and identify the defect, and then to properly correct it.

Although there could be one or more of many possible defects, you need to find it [them], and fix it in order to cause the circuit to work as it should.

Some of the things which could cause the problem you describe includes:

• A connector which has become disconnected [unplugged or loose].
• A "bad" ground caused by road film buildup and/or corrosion.
• A cut or broken wire anywhere within that circuit.
• A defective headlight circuit breaker, or relay [IF there is a relay in that circuit].
• A defective contact inside of the headlight switch.