I only have '71 wiring diagrams. On those, it looks like the second wire that serves as a ground ties into the ground terminals on the headlights (for the side the marker is on) then back to a common 'headlight ground' that ties left and right headlight/side marker grounds together.
I'm not sure why; if they were concerned that the bolted-on fenders (with sealer that probably insulates) might not provide a good ground, then the fender-top-mounted indicators might also have a ground lead; instead they ground to the fender. However Chrysler also provided for a specific ground for the headlights (per the diagram; its been years since I pulled my engine and front lighting harnesses so I don't remember exactly) so maybe thought that the other standard front end lighting should share that ground, but the optional ones on the fender top didn't get that benefit. Just a guess.