when i disassemble salvage yard doors in the bodyshop. i use the battery from my cordless tools to power the windows up and down for access to the bolts.
the seat motor would easily be powered by those batteries with jumper wires.
i would suggest doing tests with your cordless batteries at home first to be sure you are jumping the batteries at the correct terminals to power things. trying to figure this out at the salvage yard could be a frustrating process.