I dont know of any 12 v systems however one could be made if a decent radio/tv repair person tackled an existing unit.. Most electronics operate on 12v anyway within the system or just buy a converter from 120v to 12v power supply.
Now that isn't the problem. The problem is your HU card is married to your current receiver. It will not work in any other receiver. If you plug it in to another receiver you will get a call EXT 722/ display. You then plug it back into your receiver and you will get decrypted signal. DTV got a grasp on their cards when they were crackable in the past with the current HU card.
Your basically out of options except for the above.