William,
12v is available at the cigarette lighter but that would be on all the time unless it goes off when the ignition is turned off. A better source would be to tap into the 12v wire going to the Sony monitor. You could use a saddle crimp to tie into it w/o cutting the wire. Otherwise you can tap into the 12v supply to any of the dash switches that activate with ignition on; e.g. driving lights, overhead fan, etc.
Steve