You should get a lower radiator hose heater for the Benz. IIRC, $15 on ebay for a 1.5" heater and a fairly short install time.
IF you did wire up a trickle charger, you would want to wire in a continuous-duty normally-open solenoid and a continuous-duty normally closed solenoid in with the charger so that:
1. When the car is on/off and the charger is unplugged, the charger doesn't connect to the battery
2. When the car is off and the charger is plugged in, the charger will connect to the battery.
3. When the car is on, and the charger is unplugged/plugged in the charger will no connect to the battery.
Battery->N/O Solendoid(powered by charger)->N/C Solenoid(Powered by alternator)->charger
Or just remember to take the clips off the battery before starting.
Also, you should set your signature to contain you car's model so you don't have to retype it each time you post
