On the back of the deck- which deck do you have? It's a 13-pin DIN cable, should be a socket for it right there, bigger than big. Do you have the double-DIN head, or single DIN? Is this a 10-CD changer or a 6? There should be two round sockets on the back of the deck, one is for the changer (13-pin) and the other goes to the amp (5-pin).
Don't panic, if you have the wrong one, the stuff goes for a song on eBay.