As long as the basic rubber piece is in decent condition, I would recommend using a product called "Mohair Lining Tape", part# GZ-G100, 2 5/8" wide, sold by the foot, $3.95 per foot.
You use contact cement to glue the mohair lining tape right on top of your original rubber piece. This renews the fuzzy flocked part of the glass channel so the glass rides in the channel all nice and smooth.