Sounds like you may have a leak in your vacuum reservoir tank in the trunk of the car that stores vacuum for locking/unlocking the doors when the engine/vac pump aren't running. The tank is mounted up under the parcel shelf and looks like egg-crate. Check the connection where the vac line (I think it's yellow) goes into the tank. You may need to seal it up with some silicone if there's not a good seal there.

Another possibility is the leak that's causing your right rear door to malfunction is also the source of your reservoir leak when the engine is off.

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05 E320 CDI - 185K miles
82 300D - 200K miles (sold)