Trailer wiring
 

Hi. just recently purchased a 2017 GLS450 that has a factory hitch installed. or at least I think it is factory. What is missing however is the trailer wiring. There is no connector for the trailer wiring. Does anyone have experience with installing a wiring kit? Thanks for your help.

I do not know how true the following is. I went to my MB dealer last week to get a trailer hitch attached, so I could tow a trailer home with my daughter's apartment posessions. I have a 2018 GLS 550. They quoted me $5,000.00.
They told me the interior has to be rmoved to completely wire back from the dash to the hitch. That is MB does not prewire for the hitch like Ford, GM do. I said forget it.

Lee

Look on your window sticker. The hitch is <$600 option from the factory. If it is on your sticker, you already have all of the hardware, software, and wiring installed.

If the hitch was not factory installed, then what you were told is true. The dollar amount may vary by dealer depending on their labor rate, but it does involve the wiring, tearing apart the interior, replacing a module, and also software updates. Depending on the weight of the trailer you're looking to tow, there's other alternatives from Curt and also Uhaul.

1st - if you are in the US - basic trailer wiring can be installed with U-Haul for under $100 which gives you lights and brakes which uses a small relay box (normally Modulite) and splices into rear wiring.

2nd - the "factory" trailer hitch wiring which connects to the full car system is much more extensive - and can take up to 8hrs (or more) of shop time at the dealership.

For normal towing the clip-in wiring is fine - for heavy towing you can add a Wireless Brake Controller.

Mercedes has extensive features that are connected with the factory installed hitch system - to retrofit/add those connections is a real pain - there is a MB "harness" but the techs need NetStar access to see the wire diagrams - but still like 8hrs+ shop time.

Hi all, I found this thread useful so I thought I’d update it with some further info.

For those that are towing a trailer / caravan with LED lights, ours cars can’t detect it is connected because of the low power draw from the LED’s.

The solution is to wire in some resistors to simulate the power draw of normal incandescent bulbs.

I made up two harnesses, one with 5w 33ohm resistors and another with 50w, 6ohm resistors. I made up two because we have different wiring configurations here in oz and I can’t be bothered changing the terminals around - plus I wanted to try the different resistor values.

Will try it out and update the results here. The 50w 6ohm will definetly work (but will get hot). I’m not so sure about the 5w


https://cimg1.ibsrv.net/gimg/www.mbw...9ae6e5789.jpeg

Check your vin. If you have “tow hitch” as a listed item it is already wired. There is a small green dongle up in the drivers footwell to plug in a wired brake controller. There should be a 7 pin plug back by hitch that connects vehicle system to trailer.

if “tow hitch” isn’t listed in factory options you have an aftermarket hitch. Go to uhaul and they will install the 7 pin plug which solely connects tail lights. Then order Curt Echo 51180. This is a bluetooth wireless brake controller that plugs into your new 7 pin port. Works great. Caveat: as opposed to factory option, the MB will not know it has a trailer attached. Getting the factory tow option installed after the fact is prohibitively expensive. 10-16 hrs of MB Dealer hourly labor and the entire interior is uninstalled to run the wired connection. Curt wireless essentially does the same thing absent tearing your vehicle apart.