Murray and Austin did some analysis using a Method of Moments code to determine the radiation pattern of a vertical on a Jeep. The figures below show the basic arrangement of the Jeep and antenna, and the pattern, as calculatd for 10 MHz. Notice the big nulls at the back of the vehicle, as well as the pronounced lobe to the front.
Murray, K. P. , Austin, B.A., "HF Antennas on Vehicles - A Characteristic Modal Approach", HF Antennas Systems, IEE Colloquium on, 1992, pp. 2/1-2/4