I'm got cool air blowing when I turned the heat on in my 420SEL a few days ago, however it worked fine the night before, and the day after. I'm wondering if the mono-valve is going, and how I can repair/replace it, or is it possibly a bad thermostat?

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2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey

SOLD:
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino