'95 Monte rear defrost problem

I have been having some trouble recently with my rear defrost not wanting to turn on during cold mornings (when I need it most). It seems as though the relay is sticking, and its very intermittent. It will always come on after the car warms up a bit, but never when it is cold. Is there anyone who has had a similar problem? Also, I cannot locate the relay for the defrost circuit for the life of me.