Try testing the camer’s power connections. Maybe when you tested the camera using the separate monitor, the camera was getting a different power connection or source. Put the car in reverse, make sure the H/U is showing the backup screen, then check the camera’s power (at the camera) to make sure it’s getting 12V. You can also try connecting the camera to constant power instead of switched power temporarily. Some Pioneer head units run a video signal input check when they’re first turned on. If no camera signal is present, it might cause problems. Also see if there is a camera power connection on the Pioneer H/U. It might be necessary to power the camera from the H/U instead of elsewhere (such as tapping into reverse lights).