My 2005 Ford F-250 side mirror defrosters don't work?!

I just bought a 2005 Ford F-250 king ranch. And since, for some reason there's no rear window defrosters. I really need the side mirror defrosters. It has the little "de frost" symbol on both mirrors. But this morning I had the defrosters on and the mirrors never defrosted. At all! Not even a little! What's wrong? I read since there's no switch to turn them on, it automatically comes on when using the defroster. I thought maybe a fuse is blown. But what fuse would it be? I looked online at the fuse diagram. And don't see one for the side mirror defrosters. Help! Also, the truck has NEVER been crashed. And it came factory with these

Read more: My 2005 Ford F-250 side mirror defrosters don't work?!