The diagram shows it has three wires at the switch itself, a ground and two return wires back to the GEM. So I would guess that's a 3-wire switched ground (at the switch).
(I have no idea what the 2-step possibility is ???)
The output to the locks:
The polarity switches depending on if locking or unlocking (the motor goes forward or reverse) I don't know how it determines how long to run the lock/unlock motor
(the above post is based ONLY on looking at the wiring diagram, I have not ever laid my hands on these wires or done an aftermarket deal like you are)