The circuitry will be switching the green lead (in this case) to ground and then disconnecting it at a fairly high rate. If you could look at the green lead with an oscilloscope the voltage would look like the attachment below. (Electronics experts: I know I've over simplified my explanation and that the chopper power supplies I am familiar with usually chop the positive voltage, not the ground.)
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
