I would have thought you should have pretty good reception (assuming for the moment that everything else is correct) between Shayne in front and Clive behind but nowhere near the other way round as you would have both vehicles bodies in the line of sight between the antennae. I do get the impression from what both of you have said that your RF earth or ground planes could be drastically improved. A mag mount in the middle of the roof or thereabouts is likely to help with this. However large braided earth straps MAY help between the base of the antenna mount and vehicle body.