Deve, what harnesses are you using? I'm about to go that route myself.
As for the signal add on.. your original post said of the four wires from switch, one power one ground. Others don't know.
On my installation (looks to be mid 50s add on) a switched power from ignition goes to the three terminal flasher. From the flasher one lead goes to turn signal switch, this is flashing power, another from flasher goes to a indicator bulb in turn signal switch. Then from turn signal switch there is one for right and another lead for left signal lights.front and rear spliced to these leads. Doesn't involve any other lighting on vehicle at all.
