We purchased a 2008 R350 about a year ago and wanted to install a hitch primarily for accessories such as a bike rack, but also wanted the ability to tow a trailer if necessary. Locating an aftermarket hitch was no issue (we installed the Hidden Hitch 2" Class III/IV hitch), but the wiring proved to be difficult as most hitch dealers only have units (such as the ModuLite HD) that splice into the vehicle wiring harnesses. That said, we now have a working factory wiring harness and the details are outlined below.
When first inquiring about the hitch wiring, the local dealer said that Mercedes told him it wasn't a viable option to install a hitch on the R-class. The Mercedes Consumer support hotline wasn't any more helpful saying they don't have an official solution for a hitch install on our vehicle.
Not being satisfied with that answer (particularly since the manual mentioned towing in conjunction with manually selecting the gear range and the fuse table also lists the trailer and trailer brake), I called Mercedes in Stuttgart Germany and the people there were a lot more helpful. I was informed that all R-class vehicles where the VIN/FIN ends in A068595 or higher are indeed hitch compatible from the factory. They even gave me a part number for a US specific hitch package (not listed as being available through MB USA) and the part number for the Wiring harness. Note, The R-class, ML-class, and GL-class are electrically virtually identical so most parts are the same. Another note is that the Rear SAM installed in the 2008 and prior cars (as well as the 2009 model year that wasn't purchased with the hitch) will need to be replaced for the trailer wiring to work. Further, for the 12V line on the trailer plug to work, you'll need to make sure that the relays in positions U and K of the fuse box are present. Also, the R-class wiring harness is not set up for a trailer brake controller, but you can use the wiring harness from the ML or GL as they are ready for a brake controller.
Ironically, when I had the part number for the rear SAM, the local dealer called Mercedes in Tennessee and they told him that was was for an ML-class and wouldn't work in the R-class (even so the EPC listed it for the R, ML, and GL) so they refused to order it. As a result, I called the next (much larger) dealer and they had 2 in stock and installed it without any issue and programmed it (and the central gateway) to enable the hitch.
Note: the system only does not recognize the presence of LED trailer lighting, so if you need to tow a trailer with LED lighting, you will need a wiring adapter with bulb out bypass circuitry in place. Valley towing makes such adapters, but essentially all they have done is install 100 Ohm 5W power resistors across the loads for the running lights, left stop/tail, and right stop/tail.
In case you're wondering about my hitch cover, I used a Curt F-10P chrome finish metal hitch cover and epoxied a Mercedes wheel center to it.
On a side note: the hitch package became a factory option starting with the 2009 model year. That year Mercedes added a larger oil cooler primarily as a safety precaution to ensure that the engine wouldn't overheat when towing a heavy trailer in a hot climate, but this would most likely only be an issue on a hot day in the desert.
Here is all the important info in list format:
- Hitch compatible R-class
- VIN/FIN ending in A068595 or higher
- Mercedes Hitch Package
- Wiring harness
- R-class (no electric brake)
- Plug Socket
- Relays to enable +12V to trailer socket
- Hitch-compatible Rear SAM
- A 164 900 54 01 - newest model used since 11-Jan-2011
- A 164 440 46 01
- A 164 900 72 01 (super-ceded by A164 440 46 01)
- Aftermarket Receivers
- 2" Class III - rated at 350 tongue / 3500 tow
- DrawTite 75470
- Hidden Hitch 87413
- Curt C13103
- 1-1/4" Class II - rated at 250 tongue / 2500 tow
- Bulb out Bypass wiring adapters
- 7-way to 7-way cable
- Valley Towing # 39010
- VW # ZVW 808 004
- 7-way to 6-way adapter plug
- Valley Towing # 39009
- VW # ZVW 808 005
- 7-way to 4/5-way adapter plug
- Valley Towing # 39008
- VW # ZVW 808 003
- Mounting brackets for plug socket
- DrawTite 18138 - 7-way mounting bracket
- DrawTite 18140 - Short no-drill mounting bracket