Looking for advice on antenna grounding. I mounted my Comet SSB14 on a painted luggage rack crossbar and neither the mount nor antenna is grounded because of the paint coating. Getting poor performance on 6 meters especially.

Would it be better to run a ground wire from the antenna mount following the coax path, or could I ground at the PL-259 connector inside the vehicle? Radio is a Yaesu FT-8900D mounted under passenger seat with control head on console. Any thoughts on the best approach?

In mobile installations you need an antenna counterpoise, not necessarily a ground. You're actually 'bonding' pieces together, not grounding unless you're dragging a chain! One layer of paint won't greatly interfere with RF energy flowing into the car body for an efficient counterpoise.

I'd run braided ground wire along the same path as coax and connect to a good chassis ground - certainly shouldn't hurt. Ground at the source for best results. Also consider an NMO roof mount - would lower your antenna height about 5-6 inches too.

Had similar issues with painted surfaces. Use a multimeter to check resistance between mount and vehicle frame - poor grounding causes high SWR and terrible performance. Sometimes a star washer under the mount cuts through paint enough to get decent contact without major modifications.