No doubt about it the centre of the roof is the ideal spot for a VHF whip antenna but if that is impractical because of bushes, trees, low garage roof etc (or if the roof is fibreglass) another really good spot is smack bang in the middle of the bonnet ('hood' for those rebel colonists) but you will need a wire braid to ground the bonnet to the rest of the body and of course good shielded cable within the engine compartment.