This is a minor problem, I know, but I'm curious about the reason. The three knobs on the heater control panel have gone 'sticky'. I don't mean they're sticking, rather that they're sticky to the touch. They haven't been in contact with anything unusual, but it's an old car. The knobs are a sort of soft plastic - rubbery. Anyone know what's happened to them? And anyone know if I can buy just the knobs, not the whole control panel?