Hopefully I can shed some light on the story around the infamous ambulance which was used as the pre-Ectomobile.
A few years ago I was able to talk with the first owner of the vehicle, Roger, who is a really interesting guy.
Originally the pre-ecto a 1959 Cadillac Miller Meteor Futura was an ambulance used by the Bellwoods Rescue Squad No. 486. Bellwood, Illnois a suburb of Chicago.
Roger purchased the vehicle in November of 1982. Around that time he was a paramedic for a private ambulance company in the Chicago area. The vehicle was rented (not sold) to Sony for the filming of the 1984 Movie. He was also present during the filming of some of the scenes in New York City. A green light which was added to the passenger side can light serves as recognition of Chicago area emergency vehicles.
When the first Ghostbusters movie was released, Roger actually drove the grey primer car to a local drive in movie in June of 1984, and photos do exist showing the car which was seen in the movie at the drive in. Those photos were online at some point and might still be, but I don't recall where.
Roger owned the vehicle for a few years after that, eventually it was sold to a downstate paramedic and car collector- Ed? There are newspaper articles which reference Ed as the owner at one point, and he was involved with ambulances and emergency services. Later the black and grey primer car was sold and as of 2012 the vehicle was in a private car collection and in storage in Illinois, still sporting its original look from the film. I do know where the car currently is, and it is not on public display. I am purposely not including the last names of the owners because we don't need people calling them asking about the car.
The car was never acquired by or sold to the film production company (Sony) or turned into an Ectomobile for promotional use.
Vehicles that have been suggested to be the primer-Ecto are the Volo Museum Ecto-1 which started life as a 1960 M+M, and also the Star Cars Museum Ecto which may be possibly be the Mosen Ecto which was acquired by George Barris?