Yea, yank the head unit out and see if someone scrawled it on the side.

It MIGHT be that the default code is the last 4 or 5 digits of the serial number and was never changed.

Either that, or try 1 2 3 4 or the other suggestions in this thread.


I'm at a loss here - maybe some local car audio shop or dealer guys can give you some idea of what the default code would have been.
It is very likely the factory default was never changed.