https://www.ebay.com/itm/BMW-Trailer...wAAOSwi3Bf7OyN
Just FYI, I noticed today that it looks like someone is making repro wiring harnesses for the E53 towing module. The wiring harness has been out of production for many years, even though the module itself is still available new. This harness is reasonably priced and looks like it should be plug-and-play if you find a new or used module as well. (Coding may be required though)