look in your glove box and there might be a thing in there with the radio code on it.
if not you will have to take your car to the dealership or call and give them your vin number and they will look it up in their computer
2005 Mazda RX8 VR - BHR, Racing Brake, Racing Beat, SR Motorsports, AutoPower, Greddy, HyperTech
2004 Mazda RX8 TG - RIP
1997 Mitshibishi Eclipse GST - Rip