I get that message every so often, and my fix every time is to take out all CDs out of the changer, place the empty changer back and reset the car using the key... Turn the key to possiton 2 (Dont turn the car on) wait 1 or 2 second, turn the key back to position 1, wait another 1-2 seconds and turn the car on. BTW this is also how you reset your EC light if you dont want it on.
Also, I have noticed that this happens more often if i have "burned" cds in the changer.
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