Here are the issues I have seen with my tailgates:
1. The cable is not lubricated properly. Remove it, get some 3-in-1 oil, squirt it into the shaft while turning it slowly. Do this until it drips out the other end.
2. Motor is shot. That's easy, two bolts. (NOTE: My motors have ALWAYS worked, it has always been something else!)
3. Grease has gummed up. Again, easy to fix. Get some WD-40 and spray it on the arms, then use paper towels to wipe everything off - front AND back. Move the window up/down a little, get the spots you missed. Then get some black moly grease and coat all the parts that touch and slide on each other. Remember to do the wheels which sit in the sash - they are hard to get to so you may want to use some spray white lithium grease to get in there.
Some people say to use carb cleaner, but if you do not have a great breeze do NOT use that without ventilation. Those fumes have caused me permanent sinus issues and headaches, etc. It is NOT WORTH IT!
4. Be sure the cable is routed properly from the motor to the gear. It should go down to the bottom of the tailgate, UNDERNEATH the strip of metal that is at the bottom edge of the tailgate (it's just barely big enough to pass the cable through), and then back up to the gear.
5. Are all the bolts which hold the gearbox assembly present and tight? Mine were loose and would not allow me to fully close the window because it allowed the gearbox to pull away from the arm at about 1/2 of the way up and would just spin.
6. Are the arm bolts tightened down properly and making contact with the gear the entire time? Again, tighten them down accordingly.
7. The safety switch is bad. I just jump the two wires that go to the left latch with one of those blue press-on wire tap do-dads. Simple and cheap.
All in all, the glass window will be a far better choice and will last a lot longer than anything you can come up with. Yes, it takes a little maintenance, but once you get it working it should be solid.