cd changer problem

I have a 2005 Jetta GLI. I have a factory installed in dash CD player but the car is also prewired for a cd changer. I previoulsy owned a Passat and installed an after factory cd changer in the trunk. The Passat had no other cd player. When I install the cd player into the jetta the in dash radio doesnt recognize the cd changer. I get an error message showing no cd. I do not know what I am doing wrong. Can someone help or direct me to some help?

Car manufacturers are funny about their stereo stuff. It could be an Alpine headunit, and their CD changer may be a modified pioneer or sony or whatever. You really need to get the factory changer.