Hi Andy maybe I dont read it right, but if that is an E-field probe it is
not a "dipole" The input capacitance senses the voltage at two points on the
wavefront. The ground plane is, I believe, irrelevant. You just need another
capacitance separated from the "probe" to reference to voltage to. See
Renato's balanced probe (www.vlf.it)  In your case this is the car.....but
it could be the size of a dinky toy.The area of the wavefront intercepted by
the probe or the ground reference does not affect the terminal
voltage.....as I see it.................of course I might just be wrong cos
i am really a scientist not an enjuneir!!
 :-))
Alan