Not sure about on the 126, but on the 124, it's basically:
On the 124, the regulator has three inputs (well, really just two): 12v power, ground, and control voltage. It steps the current up and down depending upon the control signal. So there should be a power line (constant V) and a control line (variable V, depends on PBC signal). If the inputs are working the way they should, but the output is zero all the time, then the regulator is probably bad.
2000 C 2 3 0 Sport :: 180k