I bought my p3 from etrailer.com. On the site, they offer the pigtail that connects to the back of the p3 out - but also they have most of the major manufacturers tow vehicles that plug directly to the fuse block (assuming your vehicle is wired for towing to begin with).
Mine was plug into the fuse block, plug into the controller and away I went.
JMTandem's note about set and forget is somewhat correct. On the plus side, you can increase the initial breaking the moment your foot touches the brake pedal from moderate to very aggressive. This is extremely useful if you're doing a lot of highway driving; However, the moment you're exiting the freeway, you need to scale back the aggressive behavior of the controller or your brakes will lock up at slow speeds.
What I have found is the default setting (lowest) works the best unless your brakes are close to the Tow Vehicle limit. At that point, you do need to increase the boost.
PM me if you have more questions.
Josh