What type of car was the Ectomobile from Ghostbusters?

As one of the first comedies to use expensive special effects, and with a relatively high budget of ~$25 million, Columbia Pictures assumed that 1984's Ghostbusters was going to be a failure. Turns out, it became the second-highest grossing movie of the year, raking in over $295 million.

From the writing to the effects to the acting, it's clear that the film's success was almost entirely due to one thing: the car.

Sure, Venkman, Stantz, Spengler, and Winston are cool, but the Ectomobile played a pivotal role in the crew's patented bustin'. It was sort of its own character, reflecting the ragtag spirit of the characters in the movie and the filmmakers themselves.

Since the car played such an important role, you know they carefully vetted every possible make and model to get it just right. Something like that, anyway.

What type of car was used for the Ectomobile?