Typically, one power wire is feed from the switched position of the ignition so that when ignition is turned on, the monitor comes to life. On my '92 there is a fuse (#7) on a panel in front of the passengers seat that supplies voltage to the backup lights, the HWH Leveling and the Backup Monitor. On mine the power wire from fuse 7 going to the Backup Monitor is red BUT since I have a Sony backup monitor yours may be entirely different. Try
http://www.intecvideo.com/troubleshooting.php for some troubleshooting information.