You can try pushing buttons 1 and 6, or, 2 and 6 together. Some have said that 2 and 5 works.

or

You can pull the radio out from the dashboard (radio removal tools are required) and read the serial number from the sticky label on the outside of the case

Serial number will be in the form of an "M" or a "V", followed by a six digit number, like

M123456

V123456

If you manage to display any other numbers they are not the serial number. It must be in the form given above.

When you have the correct serial number please post here again.