Vacerator, I think you have it a little backwards.
3 prongs on power head wire to hose and 2 prongs going into the suction unit from the hose, that is headlight always on control so that power nozzle motor can be shut down. Hoover PowerMax, Windtunnel canisters did this, along with Eureka Express.
2 prongs on power head, 3 prongs going into suction unit, is full machine on/ off control on gas pump handle!! This is the general standard wiring scheme for Kenmore, Panasonic, this Royal and Hoover canisters that had on/off switches on gas pump handle for entire machine!