Mercedes-Benz 2013 M Class - Manual (Page 218)

Towing a trailer
!
Fold in the ball coupling if the trailer tow
hitch is not required. PARKTRONIC
measures the minimum detection range to
an obstacle from the bumper, not the ball
coupling.
PARKTRONIC is deactivated for the rear area
when you establish an electrical connection
between your vehicle and a trailer.
216
Driving systems
Driving and parking