Yep! Driver's side window. I found rust at the "B" pillar side of the driver's door window. I suspect that it was caused by the "gap" in the seal on the top of the door (at the bottom of the window, on the outside) that has been mentioned on this forum.
When I discovered this "gap", it was in the Summer and I was able to move the two pieces of that outer seal together (but I suspect it was too late to keep salt from getting in there). The rust started appearing last year, and I have been fighting it.
I am thinking about trying P.O.R. (paint over rust). It is available at auto parts stores.
Of course, treating rust from the outside is like trying to stop a boat from leaking from the inside of the boat.
My truck is 10 and 1/2 years old, and has lived in salt country all its life, so it did well compared to all the vehicles I have owned in the past.