Parking Sensors: No image

I just installed the Android 10 Unit (JY-FQ129N4G) in my Ford Focus MK3. Unfortunately the parking sensors at the back aren't working correctly. If I switch to reverse gear, I can hear the beeping sound but the image with the distance doesn't show up on the screen. For the front parking sensors it's working fine. I don't have a back up camera installed.

Do I need to change something in the settings or what could be the problem?$

Thank you in advance!
Do I need to connect the orange "BACK wire" to the reverse light? I thought that the unit detects the reverse gear over the canbus, but when I put the car in reverse, nothing happens.