I know! The only non-LIM reason I could think of was a short in the wiring, but I tested it for continuity (one multi-meter lead on one prong of the plug, the second on the other prong, resistance was 1 (infinite)).
The parking-mirror tilt is the other give away... it's not just a lamp issue, the truck "thinks" it in reverse (unless the mirror cues of current flowing in the lamp circuit).
They go out and the mirror returns to it's normal position when the vehicle is off or in position 1.