Stop scratching. Bad engineering or assembly construction quality.
It's a BAD ELECTRICAL CONNECTION SOMEWHERE in that particular feeder circuit
The bad connection could be either in the positive or negative side.
A cheater's fix is to rip out the incandescent bulbs by their roots and replace them with plug-in LED bulbs. This will greatly reduce the amperage load and hide the problem perhaps forever.
Not just a single wire feeds power to all the lights. From the fuses, power branches out like limbs on a tree. Problem is, all the branches and connections are hidden.
