First of all welcome to the board, now to your problem. It is not clear from your post that you are on AC settings and cool air is not coming from all the vents or you are on Heat settings and expectign hot air from center vents. Anyways I will take a stab at both, If you expecting Hot hair rather than cool, if that is the case center vent are not suppose to have any air on Heat settings. Once heater settings, air only comes out of side vents, leg area and probabally from defrost vents.
If you are on AC (cool) settings and expecting the cold air from center vents, yes you are suppose to have that. In either case it does not look like you have a blower problem, since you are getting air from driver side vent and back vents. Most probably your pods (vacuum elements) are bad, these are the one which directs air by opening and closing the vents. Do a search for the board, you will find a lot of information on this topic, pretty comman issue with older Mercedes.

Hope it helps, Good luck

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87 420 SEL
Looking for a nice mid 80's MB diesel.
87 BMW 528e (Sold)
99 TOYOTA 4Runner
1989 Honda Accord (Sold)
1988 Honda Accord (Total)