I tow a large tent trailer (3000lbs dry, 4000lbs max) and I use the Prodigy P2, as the P3 just added a fancier display and a memory for different profiles. You need the harness and as you can read above, the connector is under the dash. It was a real pain to get mine out as it was taped to another harness. Lost a lot of knuckle skin getting it free.
My favorite part of the P2 is using the "boost" profiles while on the highway. On initial braking the b1 setting applies 25% of the set braking power which helps keep the trailer in line and really adds to the overall stopping power in emergency situations. The boost settings are nearly undrivable for me at low speeds though, as even a light tap on the brakes starts hopping the trailer axle. It's very simple to change the setting while under way, so I don't mind jockeying with it a bit.