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?