probably a broken regulator at the lower bar in the system, there are 2 inside, 1 for each side of the window
http://www.autohausaz.com/search/pro...g/Repair%20Kit
Use the search to get instructions how to remove the door panel to check it before you buy anything.
http://www.bmwe34.net/e34main/mainte...oorRemoval.htm
And buy yourselves for X-mas a Bentley Repair Manual for the E34, that is a must to keep maintenance cost low and to make as much as possible DIY.
Check my website below for tips and tricks.