2001 Cadillac Catera Car dies after being off

My car was put in the shop in August because the brake lights weren't working. They said it was a brake switch, fixed it, and also put a new battery on. After that, the radio would cut off intermittently and pop back on with high volume and on a non existant radio station. Last month, the car died. No power at all and so we took it back to the shop thinking it was the battery. They said they didn't really think so, but replaced the battery anyway. Now, less than a month later the radio still cuts off when it wants to, but it also died again. If you give it a boost from another car, it starts right up and drives fine, but once you turn it off it won't turn back on. We're thinking that something is drianing the battery while it's off. But what? BTW, they checked the altenator and it is fine and we turn off the heat, lights, and radio when we get out. Thoughts?