An earlier posting said that a magmount works because the coax braid (rather
than the metal body of the magmount) capacitively couples to the car.
If current flows on inside of coax braid, and braid is an effective shield,
how can the braid capacitively couple to the metalwork of car? Either the
braid is shielding or capacitively coupling but not both at same time?

Perhaps capacitive coupling is from open unshielded body of magmount, and
coax braid adds to area of 'capacitor plate'.

However, the metal body inside the plastic cover does seem rather small to
capacitively couple to the car roof especially for HF.