I have gone through at least 3 batteries so far. Not the battery or alternator's fault. Something is remaining on and discharging the battery when the car is off. The strangest thing is that the electric windows still work when the key is off and out of the ignition. Does this point to anything? Someone suggested the convenience group relay. Hope it isn't that because it is $500 to replace. Any other ideas? It is a 1990 300E 4 matic.