ok... me thinks me has something!
looks like the pipe and assembly it connects to is for the exhaust flap on the rear silencer.
http://www.realoem.com/bmw/showparts.do?model=CD18&mospid=47502&btnr=18_0236&hg=18&fg=20
See parts 12, 13, 15 & 16. Not sure if the 'electric valve' (part #16) is hiding in the fusebox, or if they are just doing this to route it to the back? Doubt it though, as there easier ways to route the pipe to the back. :slap:
If you still have your original exhaust and your flap is not working, then this is why (I think). If not, no worries then - it's not needed anymore. :thumbsup: