p3asa - this is the harness I purchased :
http://www.carsystems.pl/vw-footwell-lighting-rear-plug-play-harness,id647.htmlOne thing to remember is that I wanted to change from the standard White LED footwell lights to put my own coloured LED bulbs in. I already had a couple of "standard" (aka filament bulb type) footwell lights in the garage from my MK V Golf days (part number 6L0 947 415) so used them with my aftermarket bulbs. If you are happy with the white light, then you can continue to use the VW LED footwell lights which obviously have to be purchased separately.
One thing though, from the descriptions on the car systems web site it said "Only one loom is needed for the fitting of both lights (drivers, passengers site)", so I assumed that meant that there was one harness and that both lights for the rear were joined together. Happily though that wasn't the case and each side of the car had it's own cable.
Running the cables wasn't hard, although I wasn't brave enough to unplug the seat wiring so instead I just removed the seat runner bolts and tilted the seat forwards and backwards as I needed to get the access I wanted without disconnecting anything (obviously with the ignition off). You need to unclip the sill trim (which just simply pulls off - just a few clips), and that then allows you access to the kick well trim on either side of the car. I didn't need to remove these fully just prise out slightly (the drivers side one would necessitate removal of the bonnet release handle anyway, and I didn't want to do that.).
So just unplug the stock connection in the front footwell lights, plug the new harness connector in instead, and then plug the factory footwell connector just removed into the female "piggy back" connector on the new harness. Then run the cable down the sills tie wrapping it to the existing harness, then near the front of the seat run the cable under the carpet to where the factory seat harness pops out from underneath the carpet. With the seat free to move and the sill trim unclipped it's very easy to lift the carpet and push the cable though. Then I just ran the cable though holes on the underside of the seat to where the cutout was for the rear lights. Unlike Kevin's expert approach, I didn't put another connection in the cable for removal of the seat, as the small amount of harness that runs though and under the seat can be very easily pulled out if required. Then just cable tie everything in place and re-assemble.
The first side took me about an hour or so, and the second (once I knew what i was doing) about 45 minutes only. Not as professional install as Kevin's but like I said I didn't want to remove the seat and mess with the factory seat plugs etc. However the cables are still invisible and I still have full movement of the seat front and back.
I hope this helps.