So I have a 91' Fox body mustang and it is a convertible so It has separate rear windows that go down in addition to the two front windows. Well they won't go down, when I push the button all I hear is a rapid clicking like they are off the track?
Welcome to stangnet, the window motors can be rebuilt easily, they likely only need the 'pucks' replaced. The pucks can be found at most auto parts stores. You can buy new motors and the vert rear window motors have an extended gear drive that's different than the door motors.
Our vendors (late model restoration is one) may have a video on the fix.
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!!