My new Factory Fit® 1966 Impala rear body harness is different from my original.

Q: Only two of the lamps on either side of the car had two wires going into the lamp sockets while the third socket only had a single wire. My original has 2 wires going into each of the 6 lamp sockets. Why?

A: Great question! We almost always build any of our Factory Fit® harnesses to the latest revision of the GM Engineering drawings.  Your car is likely an extremely early production unit. GM actually changed the design of those 1966 Impala and Caprice rear body harnesses very early in production. The original configuration included 3 totally functional stop, turn, and tail lamp bulb sockets per side. The problem was that the wiring on those circuits was not capable of handling all that current (as the front to rear flat ribbon cable on those cars was only an 18 gauge trace or feed) when both the tail and break lights were applied at one time and left on for any extended duration. For that reason, it was not uncommon to blow the tail or stop lamp fuse on those early production cars. The natural customer reaction was to install a larger or more powerful fuse. By doing that, the wiring then became the weak link in the circuit. When those circuits became overloaded, instead of the fuse blowing as it originally did, in many instances, either the rear body harness or the flat ribbon cable would melt and create a short when the bare wiring touched the sheet metal.

GM’s fix for this was to stop the heat and overload by reducing one of the turn and stop feeds on each side of the car. The lower, or tail lamp filament inside the bulb did not create as much heat, nor did it draw as much amperage as the stop/turn filament did, therefore, they were left as is. In the new configuration, strictly the inboard and outboard lamps on either side of the car now functioned as stop and turn lamps, while all 3 on either side contained tail lamp circuitry.