Radio Memory

Every time I turn off the car the memory on the radio goes away and when I turn the car back on it defalts to AM radio and I have to reset my stations again. I just got this car and it''s done this ever since I got it. All the radio stuff is stock. The previous owner has never messed with the wiring. The only thing not stock on the car is the exhaust and the airbox was removed and a K&N was slapped on there and it has a strut tower brace bar.