You have 2 choices:
1. Go to beckerautosound.com and email or call them. They can give you the code over the phone but it will cost you $20. Have your credit card ready. Or...
2. Follow this link:
Lost Radio Code - FAQ - RennTech.org Forums Send Loren a nice message. He'll hook you up with a code, but it may not work.
Actually, there's a 3rd possibility. Sometimes on these cars the code is written on the white label under the front trunk lid. That was where I found mine. Of course, that was after I already paid to get it.