I have both an 87 and 88 and the center vents definitely are used to blow cold air when the AC is on. If they don't, you have a problem with one of the vacuum valves in the dash that control air flow. This is a common problem with the 560SL--and fairly expensive to fix. Do a search and you should find many threads on this. As I understand it, depending on what valve is bad, the dash likely has to be removed. Proper functioning of the center vents is one of the key things to look for if you are considering the purchase of a 560SL, since the repair is non-trivial. Frankly, I would just drive it--it won't hurt a thing, but it definitely is not working as designed.