drivers side mirror loose

DrewE wrote:
Particularly if you were angled front down slightly, water might be able to leak in some distance back and travel forward before working its way down, depending somewhat on the exact roof edge construction. Water doesn't flow uphill as readily, of course.

Also check the clearance lights; they're a not infrequent source of angst.

I had the same problem and could not find the leak anywhere. Pulled window and resealed, re caulked front cap, clearance lights and roof several times in a year and finally found a small seepage on the vertical seal of the front cap and sidewall. I put my wife's vacuum cleaner to blow mode and blew air in that area and saw water bubbles coming from 30 ft away in the rear channel that attaches the roof to the side wall. Water was running down that channel all the way to the front because my driveway was sloped forward. After sealing I added a piece of 3/4 in oak to the inside wall and secured the mirror with 4 lag screws.