Tom, what happens is that the harness changes how the bulbs get power. In a stock rig, the power comes from the batteries, into the cab, through the dash and switch, out to the bulbs.
With the aftermarket harness, the stock wiring serves only to trigger a relay. The other side of the aftermarket harness connects the batteries and the bulb with much shorter and heavier wires. The net effect is that there's less voltage drop, thus the bulbs get higher voltages.
I built my own harnesses when I did the Sport headlight conversion. I'm still running stock bulbs. I may eventually put in different bulbs, but I'm leery about going to higher wattage bulbs in the plastic enclosures.