take of the door card and have a look,remove the motor ,usually a couple of small bolts or torx screws and check motor work as it should,if its ok which i think it will be then you may have a regulater fault,check for bad corrosion or any breaks,this is usually the problem.can be fidelly to remove but easy enough,you can slide the glass to the top and tape it in place so it wont fall or remove it,forgat to say that i would check how many volts are at the plug for the motor aswell just incase theres a bad connection.grease up the mechanism if all looks well and try again,use a little furniture polish in the window rails to help them glide
mikec
May 2010