At least you can prove to the seller that the fault lies with the HU ... yun nga lang mahirap palang kausap.
Personaly I don't think there's anything wrong with the RCA connector. If the connector is grounded or open, there shouldn't be any video signal at all. It's more like a part of the HU circuit not being able to handle the signal from the camera with whatever that affects it under different light levels.
Assuming there's nothing wrong with the connector ... If the HU is from a known brand, you may want to take it to it's own service center. Or look for the best TV technician you can find. Me, I'd rather insist on the vendor for a replacement HU.
