Stay away from these like aids. You mess with the block on this car and you're going to be wishing you were flexible enough to kick your own *ss.
Mercedes uses a coolant heater connected to the waterpump. If you want to add it, you'll need to order the following parts from Mercedes:
A 112 203 00 61 - Cooling-Water Preheater Housing
A 606 200 00 96 - Preheater
A 000 159 29 32 - Preheater Connecting Cable
A 006 997 75 48 - Seal Ring
A 210 501 53 82 - Hose Water Pump Heater Unit
A 202 501 69 82 - Hose Between Water Pump Heater Unit And Radiator Bottom
N 000000 002775 - Clamp Water Pump Heater Unit (qty. 2)
N 000000 001117 - Hexalobular Bolt Housing To Timing Case Cover M6X23 (qty. 2)
It's about $300 in parts but if you're dead set on adding a block heater to this car, this is the right way to do it. After seeing the fiasco that happened with the throttle bodies, I figured I'd post this so at least the correct information is here on the forum.
BTW, if your car is originally from Canada, you only need the Preheater Connecting Cable as the rest of the parts are already in your car.
Good luck and take care