I've had to re-build many opf these things over the years for countless cars. There are many reasons they fail - some more common then others. Here's a top down list from my experience:
Stripped gear
Bad switch
Bad Guides
Burnt out motor
Bad relay
Sometimes you can look at them and tell what's wrong. Bad guides are pretty easy to spot and so is the stripped gear. Most testing can be done with the motor installed - but to do any real work on them it's better to remove it from the car. The first thing to check - obviously - is the fuse !
If the motor doesn't seem to work at all ( no electrical activity ) then it's most likely a fuse/relay/switch or motor problem. If the motor keep making sound and nothing is happening than the guides or the Gear is suspect.