Styles, or any one else have you heard from MB yet with a fix? I went to two dealers for answers. First one assigned a nice service tech who didn't know anything about it but wanted to find out. He emailed me some ideas on where to look and none of them panned out. I found the brake controller in the right rear side. 5 wires coming out of it going to the tail lights and an unused 8 plug opening. The 7 plug is wired for a 4 plug with a hot lead. Maybe for a surge brake control? Then he quit answering me, I guess he found out the truth and was afraid to let me know.
The second dealer, the one I bought it from told be I needed to go to a trailer shop to have it wired. I let them know that I paid for a towing package with a 7 pin connector so I wouldn't have to wire it myself and reminded them when I was researching the vehicle with them I would not buy a Metris unless it was ready to tow. Then I was assigned to the service manager who said he would let me know the answer. He called later to tell me MB's answer was they don't wire for trailer brakes with a 7 pin connector, asked why not they said that's how they do things. I asked the manager if it was fair to sell me something under false pretense and he said no and there is really nothing I can do about it.
A call to MB got me the same answer, that's how we do things and there is nothing they can do, I am on my own to fix the problem.
So, I bought the Metris to haul motorcycles in, and there has been a lot of interest in the van everywhere I go with people looking at it for the same use. But I will do my darndest to talk anyone out of buying a MB product as long as I am alive. It is all I can do about it.