When i start the engine instead it doesn't work.
This can mean either: -
- The noisy 12 volt power feed to the camera is affecting the camera and making it behave incorrectly or,
- The camera video signal is getting corrupted by the car's interference but the camera is working correctly.
To solve #1 you need to filter that 12 volt feed and quite possibly use a 12 volt regulator like this: -
It is an "automotive" rated design but it will still need an RC low pass filter between the car's raw 12 volts and the input above. The R is in series and might need to be a few ohms and the capacitor is in addition to Cin shown above and should be about 470 uF or greater. I would also use a 20 to 30 volt transZorb across the capacitor in case there are voltage spikes that might exceed the maximum voltage rating for the chip (42 volts).
But this will not prevent interference being picked up on the camera's video cable. To solve this you might have to use an isolation device to "break" any chassis link that might be forcing ground currents along the video cable. Or you might just get lucky by re-routing the cable. Success will depend on what the video output stage is and what the input circuit is for the device receiving the video input. Differential is best. Single-ended has problems.
Not knowing what current the camera requires also throws another unknown into the problem.