Defrost Won’t Turn on for Several Minutes

I have a 2000 Buick Century. In the
morning, we go out to warm up the
car, and the defrost panel won’t come
on to allow the windows to defrost.
It’s not until we are driving about 20-
minutes after initially turning the car
on that the panel automatically turns
on and works. Why would this be,
and what can we do to fix it?