When you move the third climate control lever to the right, do the vents right in front of the shifter close completely?  I just replaced the foam on the flapper doors of these vents because they leaked so badly.  Now that the vents seal properly, the defroster has significantly more output.  This is about a 15 minute job.  Pull off the cover in front of the shifter and look up into the vents.  The flaps are about 1" inside the vents.  I used standard foam weatherstripping to replace the old VW foam.