Blown fuse when in defrost mode

I have a very strange problem with a fuse blowing when I put the heater select into defrost mode on my 1991 4 door. It was working fine yesterday, and the fuse doesn't blow in any other setting. Also, I have very little heat coming out of my windshield defrost vents when it worked :) Any thoughts or help would be appreciated.

Thanks

Faulty wiring sounds like, also try putting a higher fuse in there perhaps a 30?