1. Blower motor is your problem for working intermittantly. As my83300cd said on passenger side, remove cover looking up. Search w126 blower motor removal and I'm sure you will find some good info with probably pics how to do it.

2. Your center vents not blowing and cool air from defront is vacuum pod (element) behind center vents is the issue.
To change it, Remove glove box. look left hand side top inside and you will see one vacuum element. You need to change it with new one and your center vents should be working.

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1) '04 S500 Gold/Beige......120k Miles
2) '93 300E 2.8...Green/Tan MBTex..........202k Miles(and going)
3) '89 420SEL...Light Blue/gray Leather....155k Miles (Retired )