The run channel tends to shrink, and there is usually a gap behind the mirror on a 91-94, and at the bottom of the front in the 95-96 (same place, different description). It could cause wind noise too. As far as I know there are no passenger's sides available, and the driver's side is about $100 shipped from Hamilton Chevy.
Loosening the front door to move it is a royal PITA. If you slip a $1 bill between the door and the gasket...you should have some resistance pulling it out. Try it at all points around the door (window area). If not, you do not have a good seal.