One problem related to turn signal has been reported for the 2012 Mercedes Benz GL550. The most recently reported issues are listed below.
The contact owns a 2012 mercedes-benz Gl550. While driving 15 mph, the contact activated the rear driver side turn signal, but the signal failed to illuminate. The vehicle was taken to a dealer where it was diagnosed that the "light module" failed and needed to be replaced. The vehicle was not repaired. The manufacturer was not made aware of the issue. The approximate failure mileage was 61,000.