I suggest you look for a troubleshooting guide here:
System Lookup - Power Gear
Be aware that if you crawl under to test connections, the steps can jump out and bite you! Be very cautious and don't insert any body parts into the path they might travel.
Many issues arise from the reed switch and magnetic trigger on the door. The underneath connector that plugs into the step control board is another area for corrosion and bad connections.
If you decide the motor is bad, do a search on iRV2, the motor is available MUCH cheaper at an auto parts store, it's commonly used in auto electric windows.