Theory on isolation...

Maybe try a piece of rubber/insulation and use it between the rack and the antenna mount. maybe 1/8 or so thick..make small strips and add until the SWR gets to a good reading with the antenna grounded.

I am about to go shopping for my CB antenna and was going to mount it onto the roof rack using clamp hoses and a piece of aluminum.. but wanted to insulate it from scratches and isolate it from the rack.
Here is what someone posted for me a few weeks ago, and was going to try something like this

Superjens web page

Let us know how is goes and what you found.

Good luck
Joe