I just checked my stock of 69 headlight harnesses, and the side marker lamp wires are embedded in the plastic sheathing. There's no way to splice the wires inside the sheathing and leave the sheathing intact. The only way to do this is to find some large shrink tubing that will fit over the lamp socket and heat it so it just loosely fits over the wires. It can be done, but not quickly or cost effectively for my style of business. RI Wiring can probably do this well, but I've only heard results on the tail-light harness looms.
I agree w/Bob that a higher quality of repair is to offset the splices to avoid the Python-eating-the man appearance, but that is only a matter of aesthetics and more for the concours crowd. It's all a matter of what's acceptable to the individual.