faint line coming down the monitor

I just installed a 9.2" boss flip down monitor in my 2003 ford crown victoria and when the engine is running there's seem to be some type of interference, there's usually a faint line coming down the monitor. Does anyone know if I can put some type of filter between the rca video line to make it go away? or any other way to make it go away? its starting to get really annoying.2003 ford crown victoria p71 interceptor

i have seen this in afew installs i've done with roof monitors.it drove me crazy trying to figure out what was going on.it would show fine with the car off but running it had a line that would slowly come down.what i did was used a relay 30amp to make sure i was keeping 12volts to the screen.with the car running theres alot pulling power in all directions.simple hook up and they show you the wiring on 12volts relay guide.