Factory Trailer Wiring

The red/lite green wire travels back to the left side of the rear cross member. Most small/medium trailers use the combined brake & turn signal bulb filament wiring like is present on the truck itself.
 Some trailers have totally separate brake light bulbs which are independent of the turn signal bulbs. This is when the red/light green wire would be used and power would come from within the old school hydraulic type break controller. On my new school electronic brake controller there is no separate output for the independent brake lights. Some other electric controllers may have that separate output. As a side note, 7 prong RV trailer connectors don't have a standard prong position for the separate tail lights. The center aux position is usually for reverse lights. The 9 pole connectors have 2 auxiliary positions, one of which could be used for the separate brake lights. It boils down the the trailer you will be towing. 7 pole pretty common, 9 pole not so much.

kramttocs wrote

I get the trailer wiring is only with the Trailer Option but what is the chassis/flareside specific piece in the diagram that doesn't apply to stylesides (off topic likely but would like to know)?

Since the cab & chassis trucks and Flaresides used the same external taillight assemblies that were made by Grote, there's actually no ground wire. The taillight assemblies are grounded through the mounting screws to whatever they're attached to. On the Flareside, they were grounded to the mounting brackets which were attached to the bed (a really awful method of grounding if you ask me). On the cab & chassis trucks, the tallights were attached to the frame directly, or a crossmember, whatever.

The little snippet in the EVTM just shows that the cab & chassis tailights were direct grounded, and the other trucks had actual ground wires (red arrow). Oddly, the license plate lamps did have a black ground wire, but not the taillights.

Thanks both of you!