Wiring and switches not up to the task.
A relay will let you run fewer amps through the switch and supply full 12 volt power to the motor.
I would feed the relay 10 gauge and have it located under the glove box or inside the door. The old switch wiring can remain the same and be used to turn on the relay. Depending on how the switch(s)/motor(s) are you may need 2 relays per side one for up and another for down.
This is a basic answer to all rides that may have a slow motor issue caused by a voltage drop. The more information you can provide the better.