look for a sticker inside the glove box, or in the service manual. I've also seen them stuck to the spare tire or in the spare tire area.

If all that fails, you'll have to call the dealer back or go to another one. The codes are built off of the serial number of the radio and not the car. Some dealerships will keep this info and some don't.

the best idea would be to remove the radio yourself and then call the dealer ship.