how to override the security code on my kenwood?

i have a kenwood ddx814. i took it off of my wifes car, to put on mine. when i tried to turn it on, it just goes into security code. my wife and i don't remember putting in any code. so the sterio doesnt let me in. all it does is ask for a security code. can anyone please help me? the sterio is not stolen. i have serial#. its pretty old so i just hope it still works.