C – Class Battery Drain (W203 2000-2007)
A common cause of this are the seat control modules which have a constant LIVE feed even when the ignition is off.
“If you are sitting in the seat, the seat controller plug is underneath the seat right between your legs. You do not have to remove seats or anything else. Simply unplug the plug from the front floor wells and modify the plugs. The other wire in my car is a Yellow with black stripe. There are only three wires in the plug.
Each Seat Controller has a 4 pin connector that plugs into it. Remove the plug on each seat. The brown wire in each plug is the ground wire. The Red wire(Each one is a different color stripe) is wired to 12 volts all of the time. The Other wire is the wire that has 12v on it when the ignition is on. You want to cut the red wire about 2 inches away from the plug and then connect that to the ignition wire(Ignition wire is now connected to both the feeds in the plug). The main red wire that comes from the car now connects to nothing. What you have essientially done is hook the seat controller up to 12 volts only when the key is in the ignition. Hence it can not run battery down anymore because it is not hooked to 12 volts all of the time.”