With the middle button in the PBCU pushed in, are you supposed to get hot air from the footwell as well as the two dash vents (leftmost and rightmost)? I've never had any hot air from in my footwells. How does one go about tackling this one?

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver