1993 9C1 radio ?

Hi All, I installed a factory radio this weekend from a 96 caprice and the clock stays on when you remove the keys and exit the vehicle. The power does not stay on (meaning the radio does not come on with the key off), just the clock stays on. I'm wondering if the previous owner tapped into something in the circuit for the police radios that were in it. Any suggestions on what to look for would be appreciated . Thank you