1995 Chevy Cavalier Brake Lights

I have recently "inherited" a 1995 Chevy Cavalier. The exterior is in good shape, and the car seems to be in okay running condition. I did have a tune-up done; as well as had the brakes replaced. Just recently (about a week ago), someone told me that I didn't have any rear blinkers. The front one's worked fine. I had a friend look at the car, and of course he checked all of the bulbs and fuses. He ultimately changed a fuse in the box under the hood. Following that, all four blinkers work. BUT, now I have no brake lights. NONE. I have to drive around with my lights on during the day to give the "illusion" that I have brake lights. The guy at the local auto part store said that I might want to have the brake switch checked out. Any idea what this problem could be?