How do you take out a window in the lowered position on a door detached from the car, with no possible way of adding to another car to do this?

power the motor with a 14v tool battery or a spare car battery with couple spare bits of cable and clips  .
I use a 14v tool battery in salvage yard all time for testing things like fans, pumps and wiper/window motors.
On window motor you simply reverse polarity for up/down (motor drive direction)

I only have the battery in my car but ok will try that. Thanks.