I changed mine when I got my car...good thinkin' not going to the dealer..they wanted $350 just for the install!!!! (More than the cost of the window)...I bought a new one on E-bay and searched locally for a place that works on interiors as well as convertible tops...the guy installed it for like $150...
Worth it I think...The top to my car was not something I wanted to experiment with...end up with a car full of water...(or snow right now)
<img src='http://www.bmw-forums.com/forum/uploads/post-46-1094081870.jpg' border='0' alt='user posted image' />
Clear Corners and Sidemarkers
ZKW Euro Ellipsoids w/ DDE Enhanced
Debaffled Airbox w/Green Drop-In High Performance Air Filter