How can I find the code for my car radio, make "Clarion"?

I have an old Mazda with a Clarion radio, make Finish line 400, (serial number 0004837, two more numbers stamped on it are 311-1535 and 310-1447) I can't do anything with it as it displays a code number which I tried but is wrong, now I can't get it to accept anything else, even if I knew the right code, which I don't (It only seems to accept numbers up to 5). Any help would be appreciated.