Well, it looks like I may not need to haul wood inside the R63 anymore. I finally got the OEM trailer hitch installed in place of the after market Westfalia hitch. When I bought my R 63, there were two problems related to the trailer hitch: 1) the black plastic diffuser had been cut up (unnecessarily) to accommodate the after market trailer hitch assembly; and 2) the actually ball hitch would not lock into the receiver. The European hitch assembly is quite different from a U.S. receiver hitch.
Here's a photo of the mutilated diffuser:
The diffuser was cut to accommodate the electric socket, but this was unnecessary because the socket swivels down and it only sticks out so far because it uses an adapter. If you remove the adapter, the socket swivels up behind the diffuser out of the way, and when you want to us it you swivel it down and insert the adapter. A real shame.
Here's a photo of the hitch ball that is pushed up into the receiver to lock it into place:
You can't see it in the photo, but on the back of the hitch ball, at the base of the section that goes into the receiver, there is a piece of metal that points up. As the ball is pushed up into the receiver, this piece of metal is pushed down by the receiver, which releases a spring, causing the locking wheel (with the red key in the middle) to flip, which in turn causes three ball bearings at the top to pop out into a groove on the receiver, locking the hitch ball assembly in place. The cross member on the after market Westfalia hitch was perfectly straight, and while I think it would work fine on other R Class models, on the R 63 the hitch receiver is too close to the aerodynamic panel on the underside of the car and it prevents the hitch ball from going all the way up into the receiver. It flips the lock before the ball bearings are in the groove, which means that it could easily fall out.
Here's a photo of the Westfalia hitch with its straight cross member:
And here is a photo of the OEM hitch with the cross member that is not straight (it's upside down, but it makes it easier to see the shape of the cross member), which brings the receiver out enough for the hitch ball to go all the way up into the receiver and lock in place:
And here it is on the car:
I also replaced the diffuser with a different version made for R 63s with a trailer hitch. It has a precut hole to access the receiver and electrical socket (which doesn't swivel on the OEM hitch). When you aren't using the hitch, you remove the hitch ball and the socket adapter, and pop in a cover over the hole in the diffuser. Because the diffuser is black and curved, unless you are very close to the car, you can't even see where the hole is.