said by schinjay :The blower fan motor wiring is right under the fan assembly, in the open, running into the next section of the cabinet where the control panels are. Couldn't the UV light possibly be attacking that wiring?
Since UV wavelength is really short, and metal reflects it very well, the whole area gets flooded, and some UV can even leak out through tiniest gaps. That is why you must turn off the lamp each time before checking that area.
Furnace makers don't count on internal cables and components being exposed to sunlight, so UV protection is not applied. The cable insulation or internal painted surfaces could certainly be affected by UV exposure.
But my hunch is the air filter assembly, because it has plastic in addition to mold/fungus/dirt.