It could be a number of things! I had a leak in my heater MATRIX. To check this feel the passenger foot well and if its damp that could be the problem if not it could be the heater control. The left control that turns left for cold and right for hot. This operates a wire to open and shut a flap. It gets stuck.
You have to take the radio out and as you turn it free the wire off, be very careful as it is easy to break. Of course it may already be broken, then its a case of getting another one and fitting it but its not an easy job as there is very little room.
Andy D, January 2008
