1-Plug units are designed for brake controls manufactured with electrical wiring mounted on the back  on the unit. This is also know as the pigtail. Wiring harnesses must be spliced into the existing pigtail to  allow the vehicle specific plug to be installed.

 2-plug units are designed for brake controls that have a wiring port built into the back of the unit.  Simply plug the brake control end of the wiring harness into the back of the brake control and the  vehicle end into the automobile's port. Brake controls that utilize the 2-Plug style of wiring harness will  be identified with the Plug & Play logo. 

BP1 - Under or behind dash, to the left or right of the emergency brake pedal
BP2 - Under or behind dash to the left of the steering column and brake pedal
BP3 - Under or behind dash to the right of the steering column and brake pedal
BP4 - Kick panel area, on the driver's side
BP5 - Under or behind dash, behind the center console
BP6 - Under or behind dash on the passenger side, near glovebox