I have had zipper failure going on for a long time, I have never been able to repair it myself and have approached several top shops about it. The remedies recommended, remove the top, fit a new zipper/window and then reinstall the top. In the US I have been quoted $1000 - $1200 or more depending on your location. Needless to say, I haven't done it for budgetary reasons. I do plan to purchase a replacement top on Amazon and just do it myself, there are videos and written instructions from various Z3 owners on the web to help.
I have a hard top so this is not a huge deal right now. In the winter I just use the hardtop, in the summer I just drive top-down. However, I now live in Arizona and this is proving to be impractical because winter here is not what most people associate with that time of the year. It looks like I will be plopping down my Amazon top money real soon.