Well, the fix may not be 100% factory, but my mechanic showed me a 1971 Skylark/GS/Sportwagon wiring diagram (different from the one in my Fisher body manual), and it shows 2 ground wires coming off the negative battery terminal, so I guess that's why he ran the ground wire to the inner fender. Regardless, everything is working great, so the problem appears to be fixed. Even the slight glow from the GEN light I've had since day one, is gone.
