I Have A Rash OVER The Top Of Butt Crack

I have a rash OVER the top of butt crack that started out small years ago and has grown. It is ringed by spongey watery blisters and the rash itself is dry and scaley. I've tried cotizone and antifungals but nothing seems to work. It gets worse in the evening when my skin is sweaty or moist. Some of the blisters are down in my crack now right at the tip of my coxyx. The rash is currently about 2 inches by 3 inches in size and dominates the top and right side of my buttox crack.