The simplest place is often the outlet hose from the motor. This is the hose that comes from the head on the drivers side about two thirds of the way back.
The hose has no water flow when the heater is off (when working correctly). Since this is the case on this car plugging the hose will not affect the cooling system. In a recent post someone mentioned that the car ran hotter with the hose plugged. This is logical as the heater core is a heat exchanger. I have driven many a failing cooling system to help by engaging the heater full bore and rolling down the windows.
The system on this car and most MBs does not expect the cooling capabilities of the heater core to be used when the heat isn't on.
Bosch Master, ASE Master, L1
33 years MB technician