Here is a link to a Google Drive folder with instructions on how to modify a Firetruck ride-on car to be joystick activated.
I e-mailed back and forth with a FIRST Robotics team that had worked on one of these cars last year. Can’t quite remember the team number, but they seemed very enthusiastic. Are you still working with them?
Yes! Well, I work with two of the members rather than the group as a whole. They are based in Eugene, OR.