Mercedes Benz C-Class (W204) 2007 - 2014 Training Manual: Relay assignment of the fuse and relay box (SRB)

Relay assignment of front SRB

Y   Circuit 15
S    Circuit 15R
L    Spare
M   Circuit 50 (starter)
N   Circuit 87 engine
O   Fanfare horn
P    Secondary air injection pump or oil cooler (AMG)
Q   Spare
R    Circuit 87 suspension

Relay assignment of rear SRB

A   Circuit 15
B   Circuit 15 R (1)
C   Rear windshield heater
D   Fuel pump relay
E   Spare (rear wiper only for S204)
F   Partially electric seats
G   Circuit 15R (2)