The dealer couldn't find the problem (what's new). Ended up finding it myself.
I was a final year apprentice mechanic at the dealership where I worked and got a job with low beam not working. I tested the plugs at the headlights and they both had low beam power and told the foreman it's likely 2 blown globes. He told me no way; they'll never blow 2 globes at the same time so he had me strip the wiring harness from the headlights to the switch and look for bare wires and shorts.
14 hours later the following day after finding nothing wrong with the harness, I conned spare parts to lend me 2 new globes without a parts acquisition to test the headlights and miraculously fixed the problem. Then I spent another 4 hours taping the harness back up because the foreman was an idiot who would've been a more valuable staff member sweeping the floor.
Some of the diagnostic and repair processes we were told to do made no sense at all and isn't surprising that dealerships commonly struggle to find and remedy problems when the people giving the orders have no idea!