From my understanding, the whole entire back door isn't either.
The door originally belonged to the Ecto-1A and was fit onto Ecto-1 during the last restoration (as the 1A was being used as parts).
Who you gonna call? Mythbusters!
So this theory's been hanging around for a while now, and while it was considered in the topic Dr. Paul linked to, it is easy to disprove just by going back to the restoration documentary:
The main cause of confusion is that shots of Ecto-1A were mixed in with Ecto-1 during the documentary, some of which are displayed below:
Everything in the above-image is Ecto-1A*. The two top-most images show the pattern of chipped paint on the driver-side rear quarter. The middle image shows the door with the "ghosting" outline of the Ghostbusters II logo. The pattern of chips on the quarter beneath the fin that are present in the two upper images are visible in the middle image, as is the hose that's fed into the fuel cap. The lowest image is one of the few other shots to show the decal outline.
*Unlike her sister, Ecto-1A had a bracket which spanned the width of the roof, visible in the above shots, and more clearly here in this one from when Ecto-1A was at the same restoration workshop:
Finally, here's a shot of Ecto-1 when she was at the workshop:
The chips in the paintwork don't match, and the bigger clue that we're looking at two different cars is the absence of any evidence of the whip-antenna mount on the rear-quarter in the above image.
There's a possibility other parts were swapped between the two cars - though there's nothing I recall hearing from the people featured in the documentary suggesting this was the case - there is no proof the doors on Ecto-1 and Ecto-1A were ever swapped.
I agree, I don't think the car from the teaser is the original from the first film that sits on the lot
I don't see why there's so much doubt it's the original car. The scale and positioning of the rear door's logo seems to line up with other photos of the car following the 2008 refurbishment.
I've heard rumors the cast are supposed to be 12/ 13...what's the point of showing us the Ecto if the kids aren't even old enough to drive it?
Although that rumor's really doing the rounds at the moment, I don't believe anything from Jason or Sony has confirmed that so far.