You have a short circuit. Simple As.
I used to tell my students, 'You can find 99% of faults with your eyes and fingers'.
It's very unlikely that the short is inside the wiring harness where you can't see it.
Here's how I'd approach it. First, eyeball and finger every place where the harness lays against the frame. This may require cutting zipties and disconnecting connectors.
If it doesn't present that easily, use the wiggle test. Connect a test light clip to your positive battery terminal, and the point of the test light to the user side of the fuse connection. (fuse out of course) Disconnect the users, if I understand correctly the headlamps. Turning the switch off, if Euro where it completely disconnects them is ok.
If the test lamp is lit, wiggle all the harnesses until you find a place that extinguishes the lamp. If the lamp is off wiggle everything until it illuminates. That places you pretty near the problem. Then eyes and fingers. Anything more complicated than a rub-through is going to take a proper wiring schematic. ;
Edit; all of these tests with ignition key off.