I don't honestly know but the first place I'd look is to see if the door pins are sticking. Give them a good flick and WD-40 them up. Next would be the dimmer switch, possible internal short?
I know when I swapped out the dome light in my '93 with a 35 watt camper light I found power at both wires, ground was through the roof metal. IIRC, one was switched (live because the door was open) and one was constant (I figured it would be switched from the dimmer but it didn't seem to be. Perhaps possible distribution for the Explorer console or map lights). Perhaps the light's shorted out and now has constant power. Check all the connections you can safely get at. Just my .02. LR
'09 Ford Ranger FX4 that gets abused a lot... but doesn't like to swim though.