If you have tow prep (1D7 on Config Sticker) it means that the OEM VW kit will plug in at the rear and you will also probably (not always) have a module connected under the drivers seat. However the VW kit (Which is Westfalia I think) is only available with a towbar from VW. Therefore you are stuck with paying VW big money. I was quoted nearly £1400 by my local dealer for a detachable. However I Dont have tow prep. I would expect as you have tow prep it would be cheaper, as the time required to fit the electrical kit is less with tow prep.
Tow prep will save the dealer with an OEM kit about 1hr on the installation time.
The aftermarket have several options. As VW will not make the connectors available, it is irrelevant if you have tow prep or not, unless you pay VW to do the Job. I would imagine this would be less than 1k from VW.
We generally fit a full loom, which runs back to front. On older models you have to pick up connections under the passenger seat, then run to the front to pick up a the live feed from the battery, then run through a gromet in the front bulkhead to access the BCM (Body Control Module) where the CAN connections are made. This takes about 1hr for an experienced fitter. It can be done by somone who is practical with electrics, but we regularly get asked to go and sort out issues where people have attempted and failed. Of course we only hear of the failures, and im sure there are plenty of people who succeed. Im sure there will be people on this forum that have managed the installation.
Once all the connections have been made you need to update the coding on the vehicle to tell it to look for the trailer module. This will activate the towing safety features that VW build into the T6 platform. Most notably this includes trailer stability control.