Is there any documentation about your unit?
If the doc says, the input voltage is between 6-14V then you can connect it directly to the car electric system (black - ground/metal chassis and red to the +12V/positive pole).
For a try, you can use the cigarette lighter socket (central electrode at the bottom: +12V, metal rim around: grond)
If the unit needs exactly 12V, you have to buy a stabiliser (small, cheap electric unit) circuit. In that case connect cig lighter to the stabiliser and the stabiliser to the monitor.