I have seen instructions on
www.tegger.com
I have also found the instructions for Odyssey:
Anti-theft protection code on audio system
This may not work with all radios but does for my 2004 Honda Odyssey with
RES. To find the serial number without removing the stereo: Turn on the key,
make sure the radio is off. Hold down the #1 and #6 preset buttons and turn
on the radio. It will then show the radio's serial number on the display.
This only works if the radio is locked out, you cannot find the serial
number that way if the radio is working properly.
I hope this helps,
Sam