Thank you! Had the whole thing set up using jumper cables and finally alligator clips to maintain a charge on the system. It was working well (radio working), until I bumped an alligator clip and lost the connection.
BUT when I started the car, it gave me a warning about the battery replacement and the info screen told me to press and hold the power button for two seconds. Followed those instructions and the radio was reset – no need for a code.
Thanks again for the encouragement.