Pioneer Disc Changer Played on the Radio

I had a question about a disc changer my father bought and had someone else install. The disc changer is a Pioneer CDX FM1287 and it was installed onto the stock headunit for his van, the only way to get the disc changer to work is to put the radio to a certain station to oick up the signal. Is this the only way to get the disc changer to work, I have a old Sony XR C410 head unit that has the connections to hook up a amp and a disc changer. If anyone can give me information on how to connect the disc changer without the putting the radio on I would really apppreciate it.