When I bought my '91 convertible it had the same problem with the right quarter window. As stated already, it's the "pucks" (torque pins), based on your description. There are three for each motor/gearbox and they're accessed by simply removing the cover plate from the gearbox portion of the window motor. NPD sells them for something like $2 a kit.
Be aware, they may fit super tight out of the package. I sanded mine down just a little (evenly all around), and they fit fine. Just make sure you thoroughly clean out the gearbox and repack with grease before putting it back together. These things get brittle and shatter into a million little pieces! It's a lot cheaper than buying a new motor assembly!!