Since this vehicle has been so popular (not only because of Ghostbusters), I should think that that model of ambulance would have some walkaround pictures somewhere. I also remember seeing one in the old Walter Matthau movie “The Fortune Cookie”.
Well the actual maker of the vehicle is Miller Meteor and I actually found just one picture that wasn’t ECTO-1:
Those cars are very rare today - they were custom made by Miller Meteor but also other companies using chassis manufactured by Cadillac (actually different - stronger than the El Dorado chassis). There weren’t that many of them to begin with, and then they were worked hard and mostly scrapped when they got old. So unless somebody gets to photograph the one in the photo above (and it’s beautiful) this is not going to be easy!
Video walkaround of one here:
Walk-around of one undergoing restoration.
https://www.facebook.com/media/set/?set=a.267962796708024.1073741907.209759885861649
Another photo walkaround.
There was a recent repop of the 1/24 (1/25?) kit. Pretty sure you’ll be able to find even more references.