2003 Grand Am wont start and draining battery dead

I received this car free from a coworker that had been having trouble with the car not starting and draining brand new batteries to the point they wouldn't hold a charge.  The car had been sitting for a long time, she was getting ready to move and was going to junk the car. I looked at it one day and was able to start it via the remote starter.
I drove it several days trying to figure out everything it needed and parked it. After two days the battery was dead. She stated that they never had trouble with it until her sons failed attempt at installing an aftermarket radio. He junked the radio so, after reading all the post on this cars security features, I am at a loss as how to proceed without a radio. Please help.....