I believe the center outlet in the rear is for A.C. only. Check your Owner's Manual. Mine blows only during A.C. operation, and that's what my Owner's Manual says it should do.
The heat comes out of flat ducts under the front seats, blowing back into the back seat compartment.
All this makes sense, for the MB engineers know that heat rises, and A.C. falls. So these ducts are located correctly....Andras