The white plug is part of the s13 loom. It is situated near where the main plug is that plugs into the ecu. This white plug normally plugs into the car loom of the s13.Obviously as you dont have that plug in the s12 you tee into the wires that you require. If you have a look at the s13 workshop manual it will tell you all you need to know on what is what. You can view this manual at
manual pdf