NSS / backup lights question

You are correct that the 2 outer wires on the NSS are for the backup lights. They do not get power from the starter relay, or interact with it in any way other than run close to it since the middle wire of the NSS does go to the starter relay.

The wiper wires do not go thru the bulkhead connector since your wiper motor is in the cabin (66+ have it in the engine bay). There are many wiper variations. Simplest is a single speed (I think), with just 2 wires to the wiper motor (power & park) w/ gnd return thru the body. One confusing thing is that they used the wiper as a convenient wiring junction, with wires that attach there but have nothing to do with the wiper switch itself.

It sounds right that you should locate the 2 backup lamp wires that went to the column and connect those to the NSS. Polarity shouldn't matter.

To read the electrical diagrams, just put your finger on a wire and trace it. If they cross without a dot, there is no connection there. These diagrams are very simple to read, compared with say my 85 Mercedes. You can download easier ones here, with different colored wires to make reading easier, and some that isolate the wires for particular functions, like say just the battery charging system.

Perhaps the more confusing thing is that the diagrams just detail the connections, not the theory of operation nor how the components work. For example, you see the IGN1 & IGN2 wires coming out of the "ignition switch". IGN2 can apply 12 V downstream of the ballast resistor (thus bypassing it). The diagram doesn't tell you that IGN2 gets power only with the switch held in the "start" position. Even if you figure that out, you would think that IGN1 which is for the "run" position would also get power in "start", but it doesn't. You have to test that with a multimeter or read here. Actually, my M-B diagrams are a little better since they show the internal detail of each component and you might figure how the switch works if you can interpret the cryptic symbols.