any reason why a car stereo wouldnt tune into radio stations?

[missdelight]

i used to have a kenwood cassette stereo in my car & i changed it for a sony cd player stereo now i cant get any radio signal with it & yes the ariel does plug into the back properly it is not the car because it works fine & tunes in good when the kenwood is in there but nothing at all when the sony is connected so it must be the sony itself i have never had this before on any stereo can anyone help. thanks.