Rear-view mirror problem 1991 735i

I have the electronic mirror in my 91 735i. I don't know if this was standard mirror or an option. There is a plastic ring that fits over a ball that keeps the mirror in place. There is a screw in the plastic ring that enables you to tighten if needed. While loading a computer in the car I banged the mirror and now this plastic ring is cracked. I have tried to tighten it but it no longer tightens so mirror just hangs there loose. The attachment to the windshield is fine. Any ideas on how to fix this? My mechanic says I need a whole new mirror which is $258. Any help appreciated.

ebay or a breakers yard

or maybe the main dealer
http://www.realoem.com/bmw/showparts.do?model=GC82&mospid=47354&btnr=51_0475&hg=51&fg=40