1974 350SL while using the air conditioning and as the revs. drop as when entering lower speed limit the demister turns on. Even if slowing in traffic the same happens. I have replaced the vacuum hoses,only under the bonnet, and all the fittings but still the problems persist. At the moment the heater tap is tied in the off position but this is inconvenient in the rain when the window mists up. Any ideas as to my “hot” problem.