I don't have an auto. The 88 electric manual labels it the "Gear Selector" switch, but I think its the same as a NSS. The TCU reads when the trans is in N or P.
Don't let the wiring scare you. If you haven't downloaded the 88 electric manual, you should do so. Print out the pages for the wiring you a researching. Study it. Get under the truck and examine the wires and connectors carefully before taking things apart.
In simple terms: electricity flows from a power source (Bat +) through wires and connectors, to a switch, to a load (like a light), then to a ground (Bat -). The switch will either close the circuit - allowing electricity to flow to the load (light on) - or open a circuit - breaking the flow (light off). When the trans is shifted in to reverse, the switch closes allowing electricity to flow to the lights. That's why I have you test the electric at the connector feeding the switch. If you have Bat + in to the switch, but not coming out of the switch when KOEO in reverse, then the switch is bad or just dirty from 40 yrs of dirt. If power is coming out of the switch then the issue is further down the line. Use the wiring diagram to assist with tracing the wires.