That's absurd to think a used vehicle has a manufacture warranty just because a dealer sold the vehicle. No "law" states that, and dealers aren't required to supply a "warranty".
Now a used vehicle might have original factory warranty left if it's still within mileage and inservice time.
Most aftermarket extended warranties aren't worth the paper their written on, and have a 30 or 45 day wait period before coverage actually begins.
Ford? Is it a slider back glass? Either way the back glass is bolted in and labor intensive. I would call presidential glass and have them reseal it. That's all a dealer would do anyway is call a glass company out to reseal.
I would believe more that it's the 3rd brake light assy that's leaking and not the back glass.
I would hop on the Ford forums for common problems. 4951