as stated, check for air in the lines. if you hear bubbling noises, this could be a sign of air in the lines. Also as stated, this could be a damaged blend door issue. Just search blend doors and you will find tons of posts on here about how to diagnose it and fix it.
As for a wind skirt for the front. Go to your local hardware store and by some screw snaps and the associated caps that go with them. Purchase some tough material, cut to size for the grill. Might have to stitch or melt the edges of the material to keep in from freying.
Next screw the snaps into place on the grill, line your material up with the snaps and install the caps. this will allow you to install or remove the grill cover easily. I have pictures of this somewhere. I did it to my mercedes when i was in ND where it is -40 during the winter
98 ZJ 5.2L JGC Limited Sold
02 4.7L HO JGC Overland