hook an ammeter inline with the negative cable and let the car sit and check your draw after sitting 30 minutes. 203 should be about 50mA or less.

Thanks Ferdman.I tried it but still dont work.Today,my mechanic told me the car is using 0.8A (0.5A for normal car) without the engine starts.Something is draining the battery.

That is way way to much amperage draw. A 2001+ C class should draw
between 20 and 50 milliamps with the car locked after 45 minutes.
your car is drawing 800 ma. Front power seats, GPS Comand units, Phone
PSE control units and Rear Sam control modules are all common draw issues
on that car. You will need to isolate the circut with the draw.
The Prefuse box next to the battery is a good place. You can remove power
from all the main power sam/fuseboxes on the car whilst watching the draw
one at a time.
Once you disconnect the fuse box with the draw, start chasing circuts one by
one by pulling fuses and disconnecing ecus.

Good luck and remember, it takes some ecus over an hour to got to sleep and stop drawing current. (E-call and GPS)

Euro cars with stationary heaters can have draws from the heater controller
as well.

Regards,
DR.D