That light is after market. We have a picture of the truck taken in 1945, and it had no emergency lights other than the red lens in the front of the siren. However, by August 6th of 1959, it did have the light. I've seen one before on a photo of a 1949 Cadillac Ambulance from Picture Rocks, Pa. Neither the red flasher on the officer side fender, nor the light in the siren work. The siren itself works fine and proper, as does the roof light.
http://www.youtube.com/watch?v=0LeREenX4MI
http://www.youtube.com/watch?v=0LeREenX4MI