Mercedes-Benz M-Class 2012 Owners Manual - Manual (Page 311)

Infrared reflective windshield
The infrared reflecting glass prevents the
vehicle interior from becoming too hot. It also
blocks radio waves up into the gigahertz
range.
In order to operate radio-controlled
equipment, e.g. toll recording systems,
areas : on the windshield are permeable to
radio waves. In these areas, you can install
radio-controlled systems.
These areas can best be seen from outside
the vehicle by observing the light reflected off
the windshield.
Features
309
Stowing
and
features
Z