Mercedes-Benz 2013 GL Class - Manual (Page 359)

The infrared reflecting glass prevents the
vehicle interior from becoming too hot. It also
blocks radio waves up into the gigahertz
range.
To enable operation of 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
357
Stowage and features
Z