I'm restoring a 1988 Deville that was made into an El Camino type vehicle. The AC is working fine but cold air comes out floor vents only. I don't know if there is vacuum or electric switch that determines floor vents vs dash vents or if it is the climate control unit in the dash. At times, after driving a bit, the air will divert to dash vents but not always. I really need some ideas about where to start, got any?