Not sure and I cannot check out my car because its too late to go out to the garage, but if I remember correctly, the rear central vents will only blow cold air (not heated) in the same way as the front central vents do. Hot air at the rear is usually only available from the vents located under the front seats.

On the front central vents (on W140s) you can have hot air blown through them if you push the small buttom with the red dot located at the center of the vents. Maybe if you push this buttom and the system is blowing hot air, you can get also hot air from the rear central vents. I'll try it tomorrow on my car and see.

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A. Rosich
CL 500, 1998
S 500 L, 1998
E 320 T, 1995 [Sadly sold ]