Early relays were nothing but two coils around a set of contacts. One coil had the current of one rear light assembly running through it, the other the current from the other side. When current flows through a winding (coil) it produces a magnetic field. The coils in these relays were oriented in such a way that as long as equal current flowed through each coil, the magnetic fields generated canceled each other out and the contacts between them were not moved. When one coil had a higher current flow than the other, the magnetic field of that coil overcame the lower current coil's field and pulled the contacts together,completing the circuit to turn on the Pictogram. The early relays were simply too sensitive to current differences and gave false readings.
In addition to what is in the bulletin, I make sure all the bulbs are from the same manufacturer. Look at their contacts. Any greenness on their sides or melting/flattening on their ends is cause for replacement. Match bulbs side to side with equal bulbs, i.e. same brand and wattage.
Unplug the connectors going to each rear light assembly. Look at the assemblies. Note any small black marks on the pins, if it any are present, remove them with sandpaper. Look at the sockets for the bulbs. Clean any corrosion with contact cleaner, Bend the center tabs up so the bulb's tip will fit tight against them. Look at the pins in the connector for the assemblies and clean and tighten the pins.
Instead of cutting off the black ground wire, I leave it as it is and run a wire along the rear of the trunk/hatch under the trim plate at the latch plate. I slice it into the ground wires on both rear assemblies and then ground it in its center to a convenient good grounding point. This insures both rear assemblies get equal grounds and ground at the same spot.