Reverse lights only work with parking/headlights on

To test try running a separate group wire from frame to light in question. Lights can be back fed through another function and other bulbs. It is possible you are measuring the path to ground through other bulbs, and it will be low ohms.

Grounding can be a real pia. Never assume a ground is good based upon an ohm meter.

Your symptoms sound like it may be a backfield, but on a vehicle who can really guess. Lots of trial and error.