Pull the radio out (ask back if you don't know how) and check the outside of the radio itself for a 4-digit code. My used 98XC had the same problem. While on the phone with the dealer trying to get the correct code, I pulled the radio out and found the code written right on it (with magic marker!). It's not the safest place to write the code, but there is a chance that a used radio (like yours) might have the code written there.

Good luck.