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Depending on what output amperage you need, something like this Dc to Dc converter may do the trick.

Your 8V batteries are probably in some combination of series and parallel. I'd probably take two in series 16V) and use a voltage regulator to 12V socket like YC 1 said.

Are you going to use it as a lighter or as a charging socket.

How critical is it to really accomplish this?

There are dc to dc converters that can accomplish it.

Or, you could tap the second battery inline thus getting 16 volts and use an inexpensive regulator to get to the 12 volts you want.