I used half a mirror arm mount bolted to about 3"x6 plate, clamping the roof rack.
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Could not tell you why it works but it does. Electrically the rack is connected to aluminum frame so it is grounded. However I use small magnetic antenna so its only grounding thru the magnetic field. Not optimum but functions.
As far as ground 'plane' do not know whats its using. I assume its the height that compensates for otherwise poor mounting. Using everything underneath as the plane.
Most mounts the shielding sheath of coax also carries ground which is connected to base-typically mounted and its grounded.