Call this simply a WAG (Wild - *** - Guess), but when I ran a Google search for "Isuzu radio reset", a post was returned with the code number 756235 listed as the radio unlock code for a 2000 Isuzu Rodeo. Not sure how a person would actually enter that code, but who knows.

The only other search return that sounded convincing was removing all power to the radio for a few minutes (unhooking battery cable?).

Other than that, you might try calling Merlin at St. Charles Isuzu (800) 727-8066 and ask if he has any info on the subject.