Recently purchase a 2014 GL550 partly due to the 7500 towing capacity. However to my surprise... I have not been able to find a dealer that knows (actually anything) about Mercedes Brake controller options. The GL550 came with a towing package which included the standard RV style 7 pin lights and electric brake "connector and Class III Hitch" factory installed... Most States in the US require brakes on any trailer with a GVW over 3500lbs. Anyone have experience with this issue. Is there at least a factory brake wire somewhere under the dash that can be tapped into with an after market controller?

 

Fortunately Mercedes provides a very-very decent factory hitch package - which does some outright amazing things including modifyingvehicle ETS profile when towing.

As a luxury car maker - they do "step back" on towing basics/advanced - don't want to get tied up with more potential liabilities.

With factory hitch package - for you I have clipped this for you from another post.... there are more posts you can access by using forum search feature .. also see my note after.
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1) RF/wireless: You can easily add a RF/wireless brake controller to both a GL/ML with factory hitch option OR to any vehicle that has a aftermarket hitch that has trailer wiring properly installed:

RF/wireless does not require "hard wiring" the controller to the tow vehicle - the RF brake controller box gets mounted to the trailer - inline between the trailer and the harness connection to the tow vehicle - then that trailer mounted control box RF pairs to the head unit inside the vehicle - and that head unit can be cig-adapter/12V outlet powered - the controller stays with the trailer - and the head unit can be easily moved between whatever tow vehicle you choose to use - or change to another vehicle down the road as long as that tow vehicle has trailer wiring hookups. See this installation video on one of the more popular RF units.

http://www.etrailer.com/Brake-Contro...sha/90250.html

2) If the GL/ML has the factory installed tow option - then yes there is a hard wire connector - wrapped up with extra lead - under the dash - near the firewall - in the area of the OBD port under the dash - usually "up" higher on the cabin firewall. "
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If the link doesn't work - go to eTrailer for fresh link.

On "later" X166 - again with factory tow package - I "think" starting 2015 - hard wire connects often are found under the carpet.. look hard elsewhere before you pull carpet on my acct.

REALLY wireless is the way to go - easily hardwire 12V to fusebox.

 

Thank so much for the point in the right direction.. I have located the factory wire direct plug in wiring harness as you shared hanging just out of sight drive side were carpet and under dash panel meet. Just had to reach and feel a little. I really would prefer to use this method over the RF controller as I do pull three different trailers. Also at times use different vehicales. With the pairing issues and such it would just be cleaner for myself and family to have the controller internal to each vehicle... but it looks like I'm set as my GL550 has everything factory wired as you shared .. one last question of you .. I will cut into that wire harness and solder splice in wires if it's a must. However I would prefer to buy the Mercedes wire adapter pigtail with the mating connector if at all possible for a nice clean installation. I've looked aftermarket on line for hours and no luck. The controller MFG make adaptors for most every vehicle but Mercedes. Will a Mercedes dealership sell it to me? It seems like a dumb question... but my success to date with getting info from my local Mercedes dealer regarding towing was ... "a zero". Again thanks so much for the help

 

Do a search in the GLE W166 forum, I think I posted it there last year late July/early August. You can buy the connector and pins from your local MB parts dept and make your own harness. PM me if you can't find it and I can dig a little at home.