I'm not sure what you mean by all the way down. I think there should be an air gap between the headreast and the seatback when the headrest is all the way down in the vertical position so you can tilt it to the forward position. Then the headrest will touch the seatback. At least that's the way it is in my cars.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD