If you look at Ecto-1A the ladder isn't on the car at all. So im guessing in the 30 years that went by the boys went back to the Ecto 1 and just attached the ladder differently lol
The Ecto-1A has attachment points for a ladder (at least on the rack, not the body of the car) in the same place as the Ecto-1 (i.e. on the rear passenger side of the vehicle), but I don't support the idea that the Ecto-1 and the 1A are supposed to be the same car.
There are some unnecessary differences between the cars that, at the very least, don't make sense economically. Converting the 1A back to the E1 would be exponentially worse! Off the top of my head you'd have return the old hood and tailgate (or cut them up again to reinstall the louvers), replace the rear bumper, return the old whitewalls and Caddy hubs, rip out the reinforcement plates on the roof, fill all of the holes from the 1A equipment, re-drill holes for the E1 equipment, strip the decals, rewire everything, probably gut at least some of the interior, and reinstall any interior equipment that worked in conjunction with the exterior equipment. It would be a nightmare, and a monumental waste of money (though I suppose you could make the argument that the 1A's upgrades were at least partially a waste of money, but seeing as the Ghostbusters appear to be once again long disbanded, I have a hard time believing that they would have put in the effort following the events of GB2).