I am driving my fourth Mercedes ML. Fantastic tow vehicle.
Only one had the hitch installed by the dealer as part of the purchase.
The hitch costs about $350 from the dealer.
There is a wiring harness that used to be part of the tow hitch kit but last time was a separate item.
Hitch just bolts in BUT...and this is a BIG BUTTT...you must cut out part of the rear bumper plastic to allow the hitch to protrude. That is where half the cost of installing the hitch system comes from. If you cut it wrong it is a big bill. The wiring harness is just plug and play but you have to thread it thru existing holes in the body and reseal.

With all the tinkering and the risk of screwing up the rear bumper facia... I would just pay the dealer next time.