Most of the wiring for connecting to a bypass relay can be picked up from the rear light clusters apart from the reverse lights. Apart from a horrible route to thr tailgate, does anyone know if there is a simple place to tap in?

 

The best instruction I found on the web detailing the wiring connections for towbar and trailer light connection was support manuals from the towbar manufacturer Westfalia. Sorry, but I didn't save the link but a search for "Westfalia e61 towbar wiring pdf" or similar should throw something up. Try the keywords "towing module" in there.