Stock radio locked

I took the battery out of my car to change it, and this caused the radio to go into theft-lock mode. Apparently, the owner is supposed to set a radio code when they purchase the car new, but as I bought the vehicle second hand, I have looked through all the documentation that I received with it, and I have found no 4-digit code, handwritten or otherwise. I obtained the scrambled code (7913), and attempted to call several of my local dealerships, but they all insist that I bring the car in and want to charge me upwards of $100.00 to unlock it. I have explained to them that it is not in lock mode, and that I have the scrambled code for them to use. They do not seem to understand this, and just keep saying that I need to bring it in and get it "scanned". If at all possible, I would like to unlock this without forking over a hundred plus dollars for what should be a free service. If anyone has suggestions or any advice, please let me know. Thank you in advance.