I believe it is used on the fireground to identify that this vehicle has a medical (ambulance) function as well as a firefighting role. This way, members can identify it quickly if a person at the scene needs medical attention.
This photo depicts a medical emergency with a hospital ambulance in attendance, so the red light is off.
These combination fire tender and ambulance units are routinelt first due to fire calls, medical emergencies and service calls. Paris has at least one at each fire station. Because of their EMS capability, the SPDP do not usually dispatch an ambulance as well to fire calls, unlike most fire services in Germany and the US.
Les - there have been several generations of these unique Paris premier secours combination units. Several are illustrated on this site, if you look under the Renault pages. This is an older version, where the left side of the rear body was the ambulance and the right side devoted to firefighting equipment and crew seating. A very large percentage of fire calls are actually handled by just one of these little trucks with a five person crew.
Added by Andreas Hasselmann on 02 July 2013.