Copied this from Command Accessories Website:
"If you are replacing an existing third brake light, the wiring harness already includes the proper wiring. However, if you are installing a third brake light where one did not exist before, you may need a logic module. Towed trailers do not have an electrical
"signal" for the brake light. Instead, trailers traditionally use the same wire and light to indicate both a turn and a brake condition.
If your vehicle/trailer has separate red brake lights and amber turn signals, you will not need the logic module. Simply connect the new brake light to your brake light circuit.
If your vehicle/trailer uses the same red lights to indicate braking and turning, and your vehicle was not previously equipped with a third brake light, you will need a logic module for the 3rd brake light to function properly.
The logic module connects to the left and the right brake/turn signal circuit. When the brakes
are applied, a signal is generated from the logic module which is connected to the third brake light. Installation is easy and the small logic module can be located in any convenient location."
Check this link:
Do You Need a Third Brake Light Logic Module?