How can I unlock my peogeot 306 radio model PU-9990 A?

Mine is a peogeot 306 XT. The radio locked up when i changed the battery. In my search for a solution, i found this site. It is a cassette cd changer with model no. PU-9990 A and Serial no. CL0271T0000272 engraved on its side panel. The other serial nos. pasted on it are 96 241 481 ZL and 286 8497 04. I tried the 1234 method and came up with 1446. After imputing the figures, they just remained fixed on the screen. Nothing happened. I really will appreciate your help. Driving without music is like living in a world without colors. Particularly when it is ones first car. Used ofcourse. Thank you.