Dan:

The switch over valves are located behind the panel where your AC climate controls that you operate are located (M-B calls the AC controls the control panel and gently prying the panel off that surrounds the control panel and removing the temperature selector/control switch assembly will reveal the switch over valves - you will probably need to remove the ash tray and the radio to have enough room to get the panel off around the control switch off). I and other members recently wrote about working on these systems. However, that problem was with cold air not coming out of the center vents - the comments, suggestions, and opinions in that post also apply to the problem you're having. Look up that post for help.

Good Luck!
Tom

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America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.