This appears to be a common fault, I have the same problem, I have tried to trace the wiring back only to find it disappears behind the dashboard completely out of sight, I am in the process of getting the PCB checked, I find that the front side lights work but nothing else, which made me think of running wiring from the front sidelights, but when you switch on the headlights the front sidelights go out!!
Howard H, October 2021