Tou have to remove the lower dash panel. Not too difficult, but ackward as are all under dash operations.
You will find that the foam that Mercedes used to seal the AC plenums has completely disintegrated and should be replaced to insure that all the conditioned air comes out the vents and doesn't get lost under the dash. The foam that Mercedes used in this and at least the next generation of cars chemically breaks down over time and literally turns to dust. Check out your "B" pillars, for example. They originally felt cushioned to the touch; now they will feel a bit loose and flabby.

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Berfinroy in CT
Present vehicles:
1973 300 SEL 4.5
1959 Rolls Royce Silver Cloud I
1959 Ford Thunderbird convertible/430
Past vehicles;
1958 Bentley S 1
1976 ex-Max Hoffman 6.9
1970 300SEL 2.8
1958 Jaguar MK IX
1961 Jaguar MK IX
1963 Jaguar E-type factory special roadster
1948 Plymouth woody
1955 Morgan plus 4
1966 Shelby GT350H Mustang