If you are referring to that bit, it is a locomotive or low profile antenna. The horizontal slot is roughly one quarter of a wavelength, you should be able to work out the frequency band of operation from that.
It is a form of slot antenna. The radiation polarisation is at right angles to the slot where as a wire antenna has the radiation polarisation parallel to the wire.
The rear wing was an extra fitted by the body builders of the ambulance conversion. it housed street number side search lighting and rear flood lighting. I think it also helped with increase airflow to patient area.