These pushbutton units seem to be a problem area.
Mine sounds like it's shorting out, even when it's turned off. My air cycles from the floor to the windshield all by itself, and it's only about 2 years old.
These guys:
http://www.gdl-online.com/begin.html
They will do a check of the unit for $25 and a rebuilt one is about $185 with a 1 year warranty, which is what MB gives you on a new one.
Kerry- Where do I look for this valve? Does it have wires going to it? I don't think this is my problem, but I'll check anyway. My pushbutton unit is just hanging there because I've been meaning to send it in.
Craig