@Mr H I fitted the boot kick sensor today, you do definitely have to remove the whole under boot floor storage area, and getting the cable from the boot lid down to the bumper was a lot more difficult than the M3! Although, getting it through the rubber conduit between the boot lid and body was much easier than M3.
I did have an ‘oh sh1t’ moment, the plug going to the boot lid panel is different to the M3, because it also feeds the 2 rear speakers, luckily I soon realised that it can just be plugged into the rear of the boot close button, so there’s 2 cables going to the boot panel rather than 1 on the M3.