I'm not sure yet because I recently purchased our 59 Apache, but I think the trucks did not come with turn signals. Just parking lamps. Turn signals were added later. So the extra wires may be for the enthusiast who wants all original ? Ours had a burnt out front turn signal and I replaced it with a spare 1157 dual filament bulb bc thats what I removed. The turn signal on ours looks aftermarket bolted on.
Good Luck!
RK