JAEBRODY

My car has the Nakamichi premium sound system. My problem is this. I have a hard time getting the cd's to play. And once I get the first cd going, I can't change to the second one without it saying ERR. No code after it nothing. And this is on the 1990 LS 400. What do I do now. The stealer wants to charge me $700 for a cd changer. Is there any other way that I can fix this, or a cd changer that I can install purchased from Tweeter, Best Buy, Circuit City, etc?