I installed a new regulator in the rear left window. I plugged in the motor and rolled it up. But it won't roll down. I took the switch from the rear right and same thing. I bought a new motor and same thing. The master switch on the driver door does not turn it up or down, could it be an issue with the master switch, if so why?

Would help to know the year, make and model......

If you attach a test light to the harness from the motor, and your press the switch at the door, does it only light in the up position? And if you press the switch at the master does it only light in the up position?

Since you have swapped door switches and have the same result, the problem is the master switch.......the wiring from the master all the way to the motor, has to be good, since it will go up......