Are they the power towing mirrors? Only two companies I can think of off-hand that had them for our trucks were Bestop and Cipa. I had the Bestop on my previous GMC, and there was always water sitting in them after a rain or wash. Never saw rusty water, though (not before I traded it off, at least).
Only think I can think of is the mirror pane is coming unglued from the backing plate and letting water in, where it sits and rusts the innards. Maybe take it to a glass shop and have them pop the glass out of them, check for rust, and re-glue them to the backing plates.