I think that you are talking about a electric brake controller.
Brake boosters are used for power brakes on the car, or whatever vehicle you are talking about. It is the wrong term, therefore the confusion about what to install or where to install it.
Aftermarket electric brake controllers do not require the factory harness tie in. Basically they take 12 volt power from the brake pedal switch (or tap into the line going to the third brake light) and also have a 30 amp rated connection to the battery or fuse box with #10 wire and a 30 amp fuse (20 amps for some trailers). I would recommend a 30 amp automatic reset circuit breaker, so it will not trip and leave you without trailer brakes, but would reset itself after about 5 minutes.
The wiring can be buried under the carpet, just like a stereo installation, and run to the 7 wire harness that the trailer plugs into. If you did not get the 'factory' wiring package, you might be better off, because you could install #10 wire to a 30 auto reset CB on a wire from the battery to a relay that will only come on with the engine (fog light 30 amp relay will work great) to #10 wire all the way to the battery + of the 7 pin connector. Most cars and trucks that have 'Trailer Prep' only include a #12 wire, and it will not move as much power to the battery as a #10 wire will.
Good luck,
Fred.
