Ron, I'm not sure I understand your question. Are you trying to add a parallel set of lights to help detect problems? One of the issues you may encounter adding more lights is with the thermal flashers. Since they are dependent on current to heat up the element, adding additional lights could have an effect on their operation. Using LED lights might help keep the flasher circuits within tolerance.
As far as splicing goes, I prefer to redo the original style crimps, adding wires to them as opposed to using butt splice or the pinch type splices. The pinch type do not require the original wire to be cut but they tend to be a poor connection point and can be faulty. It can be a bit tricky trying to crimp two wires into a single crimp, but if you have the right tool and right sized crimp for double wires, it's not too difficult. Also, I usually try to add quick disconnect crimps on the added wires for items that I might want to remove or replace in the future.
You have to look at a schematic to look at how each circuit is grounded. On my 64, the signals were grounded through the steel of the light housing to the bumper. The headlights have a separate ground wire attached to the radiator frame and the brake lights were grounded to the light housing and on to the sheet metal. You can accidentally create a new, shorter path to ground and mess up the circuit design. I added ground wiring to all of these points to move away from sheetmetal grounding issues.
I think the best place to connect the additional wiring is to the wire harness bulkhead connectors where they connect to the harness outward to each of the lights. For the head lights, a relay might be a wise choice to limit the power load on the switch and interior wiring but this involves a bit more planning and work.
