Why does my code not work?

My car battery died, and replaced, since then the CD/radio won't work. It's a 6000 cd rds e.o.n in a Ford Mondeo.
I have the hand book which has the code 7878 but when I enter it it says wait and counts down 30 minutes (the engine has to be on) then goes back to asking for the code again. I have put it in four times now, also tried 1111 and 1234, then realised that I only have 10 attempts and it will shut down!
I have no idea where to find the serial number - there is a blank space for it in the manual.
Please help, driving in silence is driving me mad!