I have a ML320. The defrost when switched on automatically switches closed like the flap closes. Any ideas?

 

Please explain your situation little better.
If you turn your defroster ON it will stay ON, but opening and closing rear hatch will shut it OFF? Defroster has automatic timer to keep it on for certain period of time (about 20 minutes) in the case you will forget about turning it OFF, maybe this timer in your electric circuit is not working correctly.
Bernard