If it was me I would wire it off of either iginition hot wire, or the same wire that allows the radio to still work for 5-10 minutes after you power the truck off.
Since most of our trucks have dual batteries, they should be able to handle without a problem the DVD player and screen without running the system battery down during the time the radio still works while ignition if off. Keep in mind once the doors open, it will shut off anyways.
I know if I was to use a master swtich, i'd prolly forget about it and it would eventually never be used.
The other option would be to possibly pull the power from the overhead spot lights in the overhead console, however, I dont know what they are rated for in regards to amperage. I think that if you leave those lights on too long, the truck turns em off for u. I think its longer than 10 minutes though . .
My .02 worth.
Andrew Fessler
03 - 2500HD LS D/A, CC, SB
Rhino Lined, EFI Live, Remote Radar,