My '91 300E 2.6 has a new problem. I can't keep the battery charged. It is not a daily driver but I try to start it every couple of days. It will go dead overnight somtimes. I replaced the battery thinking that it was bad and not holding the charge but the brand new battery is going dead also. does anyone have any Ideas? what could be running it down? I don't think the clock that is on the dash would run it down overnight.