Mercedes-Benz GL-Class 2014 Owners Manual - Manual (Page 370)

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.
368
Features
Stowage
and
features