It should be a single wire switch and it is a ground switch. When it trips it grounds the wire out and turns the Brake light on in the dash. The in dash brake light circuit has 12v power when the ignition is on.
Edit: in factory configuration the proportioning valve is directly mounted a frame crossmember and gets the ground through that. Make sure you provide a ground to the new valve by mounting it to a grounded steel surface that isn't going to move much. Look at the paperwork you get, the valve you are using may just need a ground wire provided to one post, and the second goes to the dash using the stock wire. There should be no issue moving, extending, or even cutting the length down to bring it closer to the MC.