1. Ensure that the proper video cable labeled "back up" is connected to the correct female video cable on the back of your unit. 

2. Setting, system settings, reversing type, aftermarket camera 

    Some unit like F-150 4 inch Sync 1 even they are using the factory backup camera which need to connect to RCA should choose "aftermarket camera".

    Only like the Sync 2,3 which direct connect to the backup camera using sync choose the "factory camera"

3. Make sure the male and female connection for RCA with corresponding tab. Some car need to keep their OEM video line.