Mercedes-Benz 2011 R-Class Operator's Manual page 240

Warning and indicator lamps in the instrument cluster
238
Problem
7
The red seat belt
warning lamp flashes
and an intermittent
audible warning
sounds.
Possible causes/consequences and M Solutions
G
Risk of injury
The driver or front passenger has not fastened their seat belt. In
addition, you are driving faster than 15 mph (25 km/h) or you have
briefly driven faster than 15 mph (25 km/h).
X
Fasten your seat belt (
The warning lamp goes out and the intermittent audible warning
ceases.
G
Risk of injury
There are objects on the front-passenger seat. In addition, you are
driving faster than 15 mph (25 km/h) or you have briefly driven
faster than 15 mph (25 km/h).
X
Remove the objects from the front-passenger seat and stow
them in a secure place.
The warning lamp goes out and the intermittent audible warning
ceases.
Y page 54).