Why Mercedes 2002 C320 battery drain overnight?

Maybe you have a short somewhere leaking out the power or on the other hand the batery may need replacement. OR Try unplugging your Driver & Passenger Seat Moduals. They are a Dual-Plugg underneath each seat. Once unplugged, the seat wont operate (move forward or back) and will stay in the previous position until you plug the modual back in. I have the same vehicle, same issues.. and the Power Seat Modual was faulty. If the battery stays charged - you need to replace the Seat Moduals (anywhere from $300-500 each). Or simply keep them unplugged like I do and only plug them in long enough to adjust the seat!!!