If you bought the car used, many used car lots take the radio out and with a sharpie write it on the radio itself. The radio slides out once you release the push-in spring loaded holders on each side. You might try this before calling the dealer, but otherwise, the dealer is the only place...and you'll have to have proof of ownership before they release the information to you. Good luck!
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.