OK, A 1968 model with manual trans did have reverse lamp switch at the transmission. 2 wires went to the trans for that.
Many of will refer to one of the 3 bulkhead connectors as the wiper connector since it sometimes had no other wires in it.
Unused ports of this connector is where they put reverse lamp wires.You may find the reverse lamp wires there.
A 1974 automatic transmission soes have a 3 wire dual purpose switch. Reverse lamp wires go to its outer or left and right terminals.
A single brown wire goes to the center terminal of that switch but,,, this brown wire wasn't in a wire harness. It went from a terminal on a neutral safety switch that you may not have yet. Brown wire did join the other 2 wires sort of behind the distributer continuing on down to the trans.
Someone with a 68 manual trans example might offer pics and/or clearer details.