I've got a new theory on this. See how this grabs you.

1)I figured out the same thing as you - the roof rack isn't grounded. But it's the whole rack as near as I can tell, not just the cross-bar, although initially I assumed it was grounded.

2) Because the roof rack isn't grounded, when you attach an antenna to it, and it makes an electrical connection to the roof rack, it actually USES the rack as a part of the antenna somehow. This creates horrible SWR. Ungrounded metal attached to an antenna = more antenna? Like wrapping aluminum foil around your TV antenna years ago? Although really it's a fat hunk of metal attached only to to the mount and the coax ground, not to the antenna itself. Still, it's in there somewhere!

3) So what you had when you grounded your antenna was you had a big fat piece of roof rack in the circuit maybe, PLUS the ground?

4) I have yet to test this but I will tonight. Here is my theory on what should be done. Isolate the antenna from the roof rack (electrical tape or whatever) and then ground the antenna with a grounding wire to the frame.

Goofy? Implausible? I dunno, but I'm gonna try it.

Hed