I'm hoping someone can give me some guidance on the following:

I have a Sony in car monitor, model XVM-F65WL, though I do not have the power cord any longer so I am looking to put something together myself as Sony UK want 49.99 plus delivery for a new one!!!

The connection box that comes with the monitor (XA-126) has a 4 pin atx connection for the power cord and takes a 12V 3A input. My problem at the moment is how I am meant to connect the positive and negative/ground to the 4 pins? I currently have a cigarette lighter adaptor with fuse and cable, giving me one positive and one negative connection available from the lighter socket. I have the 4 pin atx connection and understand that the top two pins are both positive (these have yellow wires and are currently fused to give one connection) then the bottom two pins are negative/ground (black wires in this case).

My confusion at the moment is whether I have to connect one black wire to the negative of my cigarette lighter and one black wire to ground on the body of the car or whether both can be connected to negative. The original cord had a connection that was meant to be connected to the handbrake cable so that the screen switches off when you are driving, but there is a switch on the connection box that overrides this so that you can mount it in the rear of the car and have it on at all times.

Any help would be greatly appreciated, I don't fancy frying anything through trial and error.