There was supposed to be a scene in GB2 that had both cars parked next to each other in the firehouse.. The Ecto-1 and the Ecto-1a were initially supposed to be 2 different cars. Since that scene was cut, its logical to assume that they were the same car.. But they had to use different license plates on the 2 cars because they were different cars..
As a side note, they went with “Ecto-1a” in the second movie according to rumors and legend because at that time, there had already been an “Ecto-2” in one of the Cartoon GB episodes. (I dont know how accurate that is, but that's what I've heard)
TheEcto1a