Connects 2 interface.
 

I have a Connects 2 interface for the 75 with a Pioneer adaptor lead allowing you to use an aftermarket head unit with steering wheel controls. This interfaces between the ISO plugs on the back of a radio and the ISO plugs in the car (not the earlier round BMW plugs) and the plug for the steering wheel controls just plugs in.

Although this comes with a lead for a pioneer unit Connects 2 supply leads for most major manufacturers and they only costa few pounds on Ebay. More information here:

https://connects2.com/Product/ProductItem/CTSRV006.2

I am looking for £25 whiuch includes postage.