2002 Ford Focus Squeaky back brakes

I recently went to the Ford dealership because the back brake on the left of my car kept squealing everytime I press on the brake. The brake was replaced. Within 2 weeks, it started squealing again.I took it back to Ford again and was given a hard time and was told it was the weather that was making the back brake squeaking.I told the guy that perhaps the brake cracked once again but he refused to help me unless I pay out some more money to look at the problem and it was under warranty. What do you think is the problem? The back brake is still squeaking!