Classic Parts is one of several vendors that sell a wiring harness with color-coded wires of the correct length and with terminated ends ($150 - matches the original wiring and circuits). Call them to see if they include wiring for an after-market turn-signal assembly (they did with my 54/55 wiring harness, but an RPO turn-signal assembly was available in those years).
The wires in this assembly are plastic (PVC) coated. As state above, the original wires were lacquer-coated braided cloth/thread covered wires (and, a reproduction assembly costs over $600).
