I had both of the vanity mirror light switches fail in my '99 Suburban so they stayed on even when closed. Well, the switches worked fine, the part of the cover door that pushed the plunger switch down had broken off of both mirrors. Took me weeks to find it, even after I isolated the interior light circuit as the draining culprit. Then one night I had a need for something I had tucked up over the visor and was able to see the tiny bit of light leaking out from around the mirror cover door, something that was just not visible in the daylight. Those things hold two bulbs each, tiny but powerful. They'd kill a 850 amp battery in three days. Check everything that has a plunger switch, doors, glove box, under hood light, stuff like that. I also have a driver's door plunger switch that is failing. I have to wiggle it to get the dome lights to go off every couple of days. Recently the courtesy light time delay system got stuck and there was nothing I could do to turn off the dome lights. I pulled the ground cable off the battery. Once reconnected, all was well again. There must be a relay like devise in there that got hot and stuck.