I forgot to mention that my ambulance doen’t have a “behind the seat” as there was no partition in these long wheel-base 3-door ambulances. There is what appears to be a vertical mounting plate of some sort in the front side of the rear compartment, though, which I suspect is where it mounted. There are full-length fold-up seats along both sides but they stop short of the front seat and facilities for two stretchers attached to the sides and roof. Mine are missing but I know of a 1952 that has them.
than said, I have learned from reliable experts that the radio head mounted under the edge of the dash but the speaker mounted to the firewall using a threaded stud from the back.