Ok ... I've just pulled my tailgate trim apart again in order to attempt to improve the DAB situation with my rear camera. It was pretty much trial and error, I'd improve the routing go for a test drive and then look for further improvements. I've now got acceptable DAB reception on all but the weakest of stations. As a rule of thumb keep all excess harness length at the back of the roof headlining and not in the tailgate, steer will clear of the antenna amplifiers (long black plastic finger sized things on either side at the top) and also avoid crossing the wires that are soldered to the backlight glass. Here's my final routing :-