I see you're going for a largely movie inspired Ecto-1, rather than the more streamlined design they opted to use in the cartoon.
If I may make three main suggestions:
1) Perhaps lowering the height of the bodywork beneath the window line to make the model of the car look less bottom heavy.
2) Reducing the side of the spotlight on the driver-side wing, and moving it further back towards the windshield (in the movie the spotlight was attached to the driver-side mirror), and in the cartoon it was close to the windscreen on the wing.
3) The rear bumper is more triangular in shape, with the apex point sticking out in the middle.
I would recommend referring to the Kenner toy of the car, as it's a pretty faithful recreation... but that might not be appropriate in this case given the influence from the movie you're using.
I've been eagerly anticipating this project since the teaser was poster, so it's great to see the behind-the-scenes process.