Since 4 years we have a Mercedes S 55 AMG 2001 which was given to us by a friend. Since the beginning he had electricity problems. The car leaks electricity somewhere. In the beginning it was full of gadgets: fax-machine, fridge, DVD and TV player etc. We took all of that out. In the winter it is much worse - every three weeks the battery is empty. We had it checked and checked and no one can find the problem. I have a theory now (completely clueless about cars): could it be that the seat warmers use more electricity in winter than the car can produce if we mostly drive only short distances?
Is this the same battery since you got the car 4 years ago? Batteries typically have a 4 year lifespan, and they become suspect after that. Having problems in winter cinches it. Batteries have less efficiency in cooler temps, and a weak battery will be worse in winter.
Replace the battery if it hasn’y been done yet.
In addition to what BustedKnuckles mentioned, I want to point out that the electrical resistance of the seat heaters is such that they do consume a very large amount of electrical energy. Driving a car for only very short distances in the winter will take its toll on any battery, but this is particularly hard on a battery that is aging. Then, if you throw in the use of accessories that draw a lot of electrical energy, you have the perfect storm for winding up with a battery that cannot hold a charge.
In addition to the probability of needing a new battery, the OP needs to connect a Trickle Charger to the battery during its periods of non-use. And, I hope that the OP realizes that driving a car for only very short distances in the winter will take a severe toll on both the engine and the exhaust system also.
A battery is relatively cheap, but having to replace an engine in a Benz is incredibly expensive, and if the engine is accumulating damaging oil sludge–as I envision that it is–engine failure may well be the next challenge that the OP will face. The way to prevent sludge-related engine failure is to change the oil at least every 6 months, no matter how few miles have been driven in that period of time.
Guess there was a reason for it being a gift.I would think about passing it on. IMHO, there will be more issues that no one can find a fix for.