I had a fixed Westfalia swan neck towbar on our Cali that I kept bashing my shin on, so that was changed for a removable one. The bumper has to come off and the only difficulty was finding the top fixings in the rear wheel arches which are hidden under the stone shield. Once those were found it was all obvious and easy to do.
The wiring was more difficult, but not impossible. There are different wiring systems for the various ages of van so this description relates to a 2010 van.
The old wiring had been installed as a tap into the existing rear lights, so the computer was not coded. The reversing sensors beeped whenever I selected reverse because they sensed the tow ball on the fixed bar. Also, I believe the computer adjusts the braking dynamics when it knows the van is towing something, so this wasn't happening.
On my Cali the rear fog lights are disconnected by a push switch on the socket, however that may be done by the computer on later vehicles.
I decided to install a 7 pin socket from PF Jones. The splash shields underneath the van had to come off and the cable routed on the nearside over the rear suspension going past the fuel tank and the two diesel heaters and up into the engine bay. A couple of cables came up under the passenger seat through an existing grommet and a control box was installed in the battery compartment under the bonnet. Two more cables then went over the back of the engine attached to the bulkhead and down to a grommet in the bulkhead by the steering column. These passed up under the carpet behind the dash and were tacked into the Can Bus system behind the light switch.
Routing the Can Bus cables was the most difficult bit as I couldn't get underneath the van easily to see what I was doing, so it was all done by feel. Much cursing and scratched hands later, all was good. My local independent garage re-coded the system for me and all has been well since.
If you are a handy DIYer and have the time and space on a drive, then doing it yourself is perfectly feasible. If you feel daunted by this then I would go to a Westfalia specialist and get it done by a professional.
Since then I have bought a BackBox with a 13 pin plug, so I wish I had installed the 13pin system and not the 7pin I chose. However, it was perfect for towing the vintage trailer tent my daughter bought.
Good luck.
Alan