Just a bit of info, my new donor has a badly cracked rear window which needed replacing. Fortunately, the roof is an original BMW roof so the window is a zip in, aftermarket roofs have a sewn in rear window so much harder to repair.
The rear window was purchased from EBay for £71 +£6.50 P+P, it is listed as an American company but they have stocks in the UK and it only took 4 days to arrive.
http://www.ebay.co.uk/itm/BMW-Z3-Con...72.m2749.l2649
It took about 40 minutes to fit by myself but would have been quicker with 2 people.
Just push a pen thtough the old zip to remove the window and then prise the small metal pate off where the zip ends meet.
Then place the zip tang they supply onto the new window, it is easier this way as the new zip doesn't have a lead in. The lead in on my roof was missing so it took a bit of time getting the zip into the tang. I put some super glue on the material where the zip lead in should have been. This stiffened the material up making it much easier to thread into the zipper tang.
Position the window where you think it should be, as you get to the first corner you will see if the window meeds moving to the left or right, re position as neccesary. When you get to the first corner again you will see when it is all lined up, at this point it will be hard to work the zip around the corner and you don't want to damage the zip. Pop the front of the roof open and prop it open by about a foot. This realeases the tension making it much easier to work the zip around the corners. Close the roof and make sure it is in the correct position with no wrinles. If there are wrinkles, they won't go away, remove the window and start again.
Finaly sow the two ends of the zip tpgether with the supplied needle and thread. Then glue down the edges.
The one thing the replacement roof doesn't have is the plactic channel that the edge of the existing roof goes into.
I had a new window sewn into an E36 original zip on a old car I owned. It proved impossible to get the edge back into the channel so this way although not perfect is probably the best option.
40 minutes and £77 later