The holes above the windshield could have held a speaker like this...
A radio head like this would have been mounted below the dash, the main radio box would have been very large (approximately 16"L x 12"w x 4"d) and could have been under or behind the seat.
Check out these web sites to see all the different sirens. If you can't find a vintage picture of your truck you'll need to measure you base holes and see what might have been bolted on the fender as the bolt patterns are all different. Small local fire companies didn't have money for big Federal "Q" sirens, they used something smaller or made by another manufacturer like Sterling or Sireno.
http://dcaptain.com/vintagesirens.htmlhttp://www.sirenman.com/Mike B
