Once you have tried all the above. Try a totally separate power supply so the camera is not connected electrically in any way to the car. It may help narrow down issues.
Higher quality properly shielded cable may help. This was an issues decades ago with RCA hi-fi cables in my car. I bought top of the range seriously expensive Van Den Hul cables, interference solved perfectly. The cables had multiple shielding.
Try a camera from a different manufacturer, maybe you can borrow one off a friend for a test, & different cable if possible, separately tested to narrow component issues.