A solid state 12 to 6 volt reducer would probably be best. The wire wound ceramic reducers won't heat up enough to reduce the voltage by much in the short time you'd be using the seat motor. That said they'd prolly work just fine on 12v. The seat will just move 2X as fast as normal. My Merc has 6 volt factory power windows and they work well on 12V. Just keep appendages out of the way when closing them.
They are the same motor as your seat motor.