I don't remember what year ML donated the towing kit that's mounted on my '98, but it had a module that plugs into the harness to provide the brake/turn signal isolation functionality. I've recently pulled the towing kits from two 99s, and both of those harnesses just have a flat board in the middle of the harness covered in heat shrink material. I assume that whatever circuit board is in there performs the same function as the more separate module on my first harness?
Towing kit wiring harnesses
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The style of harness I pulled from the two 99s looks like the one in this thread:
www.benzworld.org
The style of harness I have (which I think is post-2000) looks like this picture I stole from eBay:
![80607E5C-44AF-4C2A-B8E3-FC993AA7E3C8.jpeg]()
I have the 1998-99, 2000-02, and 2003-05 towing wiring diagrams. All are pretty much the same in that area, which is why I was surprised to see the large physical difference.
Found an OEM Trailer Hitch - Ready to Install
After failing to find a used trailer hitch for my 2002 ML320 online or at the local salvage yards, I spotted a 1999 ML350 with a hitch installed in the parking lot of the local Barnes & Noble bookstore. It didn't look like it had been used recently. I left a note on the windshield offering to...
www.benzworld.org
The style of harness I have (which I think is post-2000) looks like this picture I stole from eBay:
I have the 1998-99, 2000-02, and 2003-05 towing wiring diagrams. All are pretty much the same in that area, which is why I was surprised to see the large physical difference.
Are you talking about the plug in unit on the side of the harness? It’s an isolator module to keep shorts in the trailer lighting or connector from knocking out the matching lighting systems on the vehicle side. My assumption is that the circuit board in the shrink wrap is the same thing, judging by the wiring diagrams.
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