This was the last car I didn't have as well, and I futilely tried to find it from Hill St. and 4th parking lot for a while.
However, I then found a suggestion in the comments of this Youtube video:
Take a Ford H-Boy (one of the hidden cars, find it on the map) and drive it up and down Catalina Street. Had a Zephyr spawn in oncoming traffic after about two minutes (it has a fairly distinctive front grille, look for it), drove in front and commandeered it. All done.
Probably doesn't matter, but I was in the Arson Desk free roam when I got it.