Which camp are you in - Taxi Driver Ghost was a ghost or a possessed
taxi driver?
I think the implication is supposed to be that a taxi driver was sitting in the car, a ghost entered through the exhaust pipe, and possessed the driver.
The only time physical non-human beings are seen in the first movie are Gozer, the terror dogs, and the creatures in Dana's chair, all who I assume to be flesh and blood aliens/extra-dimensional beings, and not the spirits of dead people.