This may well be a total red herring but copying the BMW number off the hanging down harness connector - which is very difficult to read, but could be 1-967-415-1 and Google that comes up surprisingly here:
https://the75andztclub.co.uk/forum/showthread.php?t=632. This leads to a lengthy debate which may suggest it is a cruise control lead and nowt to do with the radio. What do you think?