How can I get code number to reset my radio?

Replaced my battery in my car and now the radio stopped working. Dealer says I need to put a code into it and reset it. I bought the car used and I have no code number. The dealer says that I need to remove the radio and get the serial number and then he can give me the code for that radio. Is there any other way to get that code besides pulling the radio out of the dash???????? Thanks for any help.

2001 Mitsubishi Eclips Factory Radio