Locked radio

I'm selling my Saxo (having just got a C5) so this afternoon went to re-fit the original radio having removed my aftermarket stereo.
When I connect the radio up and attempt to turn it on, I am faced with
two messages from the LCD display:
code 1 error
I am assuming this is because the stereo requires me to input the code I have written in the vehicle handbook. I have no idea how to input this code, however, because the four digit code includes a zero and there are only buttons from 1-6 on the facia!
How does one go about unlocking such a thing?
--
-Toby, who...
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Toby Newman ( snipped-for-privacy@asktoby.com) gurgled happily, sounding much like they were saying :

If it's like the Philips stereo in my s2 XM, then you use the tune up/down buttons to take each digit higher/lower, then the "1" channel button to store that digit.
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Thanks for the swift response. That wasn't quite it but it gave me an idea of what to try. I had to wait for the 1 error code to go away and could then input the code using the 1234 buttons to change the first to fourth digits.
Success!
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