Decrease torque of an electric motor
Just too little information to write a real answer to this. 

Reducing the voltage will indeed reduce the max current and speed of the motor that is driving this. However, if it is indeed current sensing to stop the drive if the antenna is obstructed, reducing the voltage too much will defeat that function. 

Best way to find out is run it off a bench power supply, adjust the voltage down from 12v and see what happens. Of course, if you reduce it too much, it will not move at all.

What are you using it for anyway, I'm guessing not as intended.