[27litres]

I haven't done a D3 before, but I have installed towbars on a number of cars.
The most relevant is the D2.

The official wiring kit for the D2 is obsolete, and can't be found.
I'm not sure how advanced it was in relation to the ECU/ESP, but I expect it would disable the parking sensors, which mine currently doesn't and is a pain whenever I reverse a trailer!

If you can get a factory loom for a D3, I suggest you do, as manufacturers quite often do a lot of work on towing kits to tie them in to things like the stability program and parking sensors.

If you can't find one, you will need a relay box - these are small and inexpensive and can be had from towbar installers and caravan places. They are needed to keep the trailers' electrical drain off the lighting circuits (thanks to bulb warning). You will need to find a suitable ignition switched live circuit and tap into it (dual fuse taps are good for this).

You will need to tap into the lighting circuits at the rear of the car. The D2 has a great feature here in that the lights themselves have spade terminals for each of their circuits making tapping the lights extremely simple!
I'm not sure what you use your extra pins on your trailer plugs for (we only have 5 or 7 pins on our plugs), but you will then need to tap into any additional circuits as required and couple to the relay box.