How can a CD player be installed installed in a Nissan Almera?

I have a Nissan Almera, circa 2002 with a cassette/radio fitted. There is space for another device and I wish to fill it with a CD player. However, I want to keep the existing system alongside the CD player. However, Halfords told me that the cassette player/radio would have to be disconnected in order to connect the CD player. Is there any way to connect the CD player without disconnecting the cassette player, i.e. by using adaptors or 2 way connectors etc?