Just to add more to the possibilities. If memory serves correctly many people who were doing this on the OBS powerstrokes back in the day were using a valve from a ranger. It was vacuum controlled and could be tied right into the existing set up. So when you selected cold it shut it off. Hot it opened.
I never got that fussy about it. It was crazy hot in Boise when I lived there 9 months a year. Once I moved to NH I opened it and never needed to close it.