First, you need to drive somewhere cold.
I recommend Ushuala, Argentina.

Then, let the car sit over night, after spraying the car down with cold water, making sure to get a lot on the rear window.

Then try the defroster with the engine running, and see if it works.

If it doesn't work while you're there, post on here to let us know, and we will let you know what to try next.
You don't need to come back to the States until after we get it fixed.

BC.