As European models are not that luxurious that often, I know there are some differences in heater systems (even in the current E-class and W202 C-class).
If mercedes builds a car with a heater only (no airco or climate control) the center vents are only used for bringing in fresh air and are not able to bring in heated air at any time(as in zachv's W123/with airco?).

However if the car has got an optional (probably standard in the US) airco or climate control, the center vents can bring in heated air if the heater is in use (in combination with fresh air from outside) and the heated temperature floating from these center vents IS not as high as the temperature coming from all the other vents.
Probably a bad vacuum pod; and I think this specific pod is situated somewhere on top of the heater housing (as with my C-class) and not easy to reach (removing the complete dash).

greetingz,

__________________
1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)