X2 on Etrailer, thats where i got mine and the plug in harness. Get the harness that plugs into the rear end harness in the rear passenger corner, thats the easiest way to do it.
http://www.etrailer.com/pc-VWH~42475.htm
You just pull up the trim piece across the tailgate opening and the remove a couple screws and pull out the right rear corner of the interior panel. Unplug the stock harness and plug your trailer adapter inline and then put it down through the floor grommet that is there for the factory trailer wiring. I just cut a slit in the rubber plug and pushed my trailer plug down through and then sealed around it with some RTV.
The bolt holes make look clean but you might want to spray them with some oil and run a bolt in and out each before putting the hitch up in place and then find out you have a problem getting them to run through. I took some PB Blaster and a plumbers brush and cleaned each one out and then run a bolt in and out each of them.
Also on the rear passenger side, you might want to consider a jackstand and remove the wheel, the front bolt on that side is above the exhaust pipe and not accessible from underneath, but if you remove the wheel you can go in through the wheel well and there is enough room to swing a ratchet or wrench. I had to use a wrench to start it until i had enough room to get a ratched on the bolt. Wish i had one of those ratcheting box wrenches that size would have made the whole thing easier.
As for the ball mount.... I have a 3" drop and no problem getting most couplers on it. Etrailer does sell some nice long ball mounts, and they are rated as high as a shorter one, when you put in an extension you loose a lot of your load capacity. Might want to consider ordering an extra long ball mount if your worried about that. I just swing the gate open and hook up and then swing the gate closed if i need to.
http://www.etrailer.com/p-80232/Bal...se-or-2-Drop---Extra-Extra-Long-6000-lbs.html