As Mike has said above, have a good look at the original wiring diagram and the various posts regarding the modification of moving the indicator flash to the reverse lights. The original wiring (on a 68, but I presume is a similar concept for other years) means that the brake light signal feeds into the indicator circuit at the dashboard relay. This needs to be removed, and a new cable run from the brake light switches to the tail lamp. The indicator wire can then just be moved from the red lamps, up to the ‘reverse’ lights.
I think that is summarised correctly?!