I just got finished replacing the drivers side actuator. If it wouldn't have been for this write up, it would have been a heck of a lot harder to do. I just want to say 'Thanks' to the OP and everyone elses additional posts. The vehicle was a '06 325XI (E90)
I bought the part on an internet order from Tomkinson BMW Parts.com. They substituted the part with the newest part (p/n 51-21-7-318-421 207D). The part cost me $57.98 with $11.95 shipping.
The part that I found the most frustrating was also the plastic window clips. Mainly because I was concerned that I might break the glass prying against it trying to get the clips out of the windows holes. Never-the-less, I was able to get it apart and reassembled with virtually no problems.
Thanks again for posting this info.