back up light fix

Hi guys I've got a 71 swinger with a 73 dash. Some wiring is the same some different. one mess at a time and I'll get them all Now to the backup lights. I notice that the neutral switch wires and backup light wires are two different gauges. Does the N.S.S power a relay or supply a ground for a relay?The center wire grounds the starter relay,do the two outside wires complete a circuit for the backup lights How do they work? Thanks Jim