Hi - I can't trace it through for you right now, but you may be helped by looking at the '73 diagram on the Tom Collins site. A '73 has a fuse panel, but I would expect the wiring to the back half of the car to be pretty much unchanged.
My other suggestion would be to trace back the wire in question to its origin, and infer it's purpose from that.
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
ECO Green: '15 FCA Jeep Cherokee KL Trailhawk