Rear Window defroster blowing fuse on wet days

Folks,

My rear window defroster is blowing its fuse on rainy days. During dry time, I can turn it on and leave it on with no problem, but when it rains (when you most probably will need the defroster) the moment I hit the switch, the fuse blows.

I know the switch isn't getting wet, so I must assume that the problem is elsewhere. Can it be on the rear window itself?

Any ideas would be greatly appreciated.

Rgds