Mercedes-Benz 2013 GL X166 Operator's Manual page 359

Features
357
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.
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