My rearview monitor came with 3 connectors on the cable - White, Yellow and Red.
The Yellow and White RCA connectors are both video inputs - you can connect the R-Pi composite output to either of these.
The Red connector is a barrel jack for power - it requires a DC power source which can be any voltage between about 9V and 15V - where the outer barrel is ground and the inner pin is positive supply voltage.
The display doesn't do anything until power is connected and there is a valid video signal on one of the inputs.
