I put a hitch on my Mercedes E350. It's nearly invisible, and came complete with the electrical interface that keeps the computer happy. Highly recommended. 
Here's the link for the ML hitch:  
http://execuhitch.com/tow-hitches/mercedes-gl/
-John, Q3
On Thursday, April 2, 2015 at 4:03:04 AM UTC-4,  wrote:
 I was quoted $3,500 to $4,000 by the Mercedes Benz dealer to install a factory tow hitch on to a ML 350 Bluetech 2011!  I was wondering if anyone else had this happen and if so, where did you go to have the work done and were you happy with it?  I live in Southern Cal.