You gotta pull the door panels. There's a rubber seal with a felt backing on the top on your door panel. You have to inspect the backside of that seal. Some of those have staples holding them to the door panel. Could be a backed off staple or something stuck in the felt. Plug the window switch back in with the door panel off and operate the window while looking in the door to see if anything else contacts it. You should be able to find it.
I had the same problem with my 05. Late Model Restoration sells replacement pieces to fix the problem for a reasonable price, around 40 each side. Here is the description of the part look it up, well worth the money.
2005-09 Mustang Lh Inner Door Belt Weatherstrip by Ford