Better Idea!

Place a dongle on the serial port. Have the code look for the dongle on startup and if it sees it then do nothing. If the dongle is missing then only display the following and not allow the unit to operate at all.

THIS UNIT STOLEN FROM
richard head
Please call 123-123-1234
Reward for return

The dongle can be just pulling the DTR high on the serial port, or a basic stamp that issues a response from a challenge. bury it in the dashboard, Voila. Thieves are pretty stupid (They're thieves, they arent smart enough to get a real job) and they wont go looking for hardware buried deeper in the dashboard.