Not sure which box the 6 speeder was but I do know the 850CSi had it as an option, one of the members here down under has one.
Did a quick search on the online ETK (yeah I love tabbed browsing

) and the box is in the diagram below
http://www.realoem.com/bmw/showparts...10&hg=23&fg=05
obviously more to it than that, but the rest is left as an exercise to the reader
Cheers,
Iain.