Federal Interceptor Siren
A 1972 Cadillac ambulance would have almost definitely had a Federal Interceptor (PA-20) 100 watt output siren. The Federal Director (PA-15) was the precursor and was only 58 watts output. Not desirable for a high class Cadillac. The PA-150 is a much newer siren. I do not think it was even manufactured in 1972.
I would call Doug Kelley now. I spoke with him yesterday AND HE HAD JUST TAKEN IN A NICE PA-20, so now would be perfect.
The Motorola Radio would usually be a multi-channel Motrac, Mocom-70, or Micor (high $$ for 1972 since they had just come out). There was an EMS application for all three of these models which included a remote speaker, microphone, and control head (often different from the unit in the front) which was located in the patient care compartment. These are rare but can be found or "fabricated" from existing pieces. Some of them were just a speaker with a microphone built into it and the frequency and volume was still controlled by the front control head.
All three of these models are readily available on e-bay. Just get the Control head, speaker, front control cable, and microphone. You can hot wire the green light to come on with the ignition, cut off the control cable after tunneling it up under the dash, and it will look completely original. The box was always buried either behind the front seat or under the floor in the patient compartment.
No one would ever see it except the radio tech.
Good Luck.