Hi, I have a 2004 Corsa which is generally doing a fine job. However, the rear window heater/demister only seems to work properly for the top half of the window. The mist clears quickly around the wires on the upper half but does nothing at the bottom. Has anyone else had this or know what it might be? Might the upper and lower half of the heater wires be on different circuits or something? Thanks, Tom