I have a 500 SEC, 1984, European model. When the car is at idle the heat and a/c, including it seems fan speeds, work. As soon as I get up to highway speeds, I get neither heat or a/c. The fans blow but no moderation of air temperature. That years dash has a fuel efficiency (a vacuum ?) gauge on it, and it seems to me that when the needle goes up, the heat/a/c stops.