Connecting CD player with factory casette radio

My 98 pontiac has the factory cassette/radio player with theftlock installed but it didn't come with the remote CD player. Against the sunlight on the radio display, at the bottom left hand corner there is a symbol that sais CD, so I decided to try to install one. I had a CD player from another car and I installed it into this one via the extra unused jack in the back of the radio. Now when I put in a CD into the player, the radio doesn't play it back. For example: when I'm listening to the radio and I put in the CD, the radio is still playing. Usually when the disk is inserted, the radio is automatically supposed to switch to CD mode. There is power to the remote CD player and it does function by ejecting the disk when Eject is pressed, but the only time the CD Player doesn't work is when the key is in the accessory position. Why is there power to the CD player, but the radio does not play back the content on the CD.