I am only a chick and not a car buff, but they sure look like ponty knockers to me.

In the color seasons Steve drives Pontiac station wagons (one may have been a Catalina), and beginning around Season 8 Robbie & Katie drive a big Pontiac convertible. In Season 7, the character of guest actor Steve Franken drives a gorgeous Pontiac GTO convertible. I've also verified that in the Season 6 ep with Sherry Jackson, Robbie's rival is shown with a beautiful GTO convertible.

I think one of the reasons they had Steve & Barbara honeymoon by car in Season 10 was to show off their nice blue station wagon. Likewise in that season Rob & Katie have a scene washing their convertible, which looked similar to one Katie learned to drive in a couple of seasons earlier -- but possibly a newer model provided by GM. I'll have to recheck the early eps of Season 10 to have a look at Barbara's car, which apparently she no longer needs after moving into the Douglas household.

I think there was continuous GM sponsorship, obviously the earlier seasons were sponsored by the Chevrolet division.

I wondered that..... They all look the same!!

Okay, when Steve and Barbara have their pre-wedding argument (Season 10 ep 7), we see Barbara's big blue 2-door Pontiac convertible. From my research it looks to be a 1966 Bonneville, which was a few years old. Nice car. (2-door cars & convertibles tend to be more highly desirable among buyers of vintage cars today.)

There's also someone else's Pontiac parked across the street in that scene.

A couple of episodes later as I mentioned before we see Steve's station wagon -- but I can't pin down the model. It looks as though Steve's earlier blue Season 6 station wagon likely was a Bonneville model. There were so many models.

The first four seasons Chevrolet sponsored the show so they only drove Chevy’s then. Sometime in season five Chevy dropped sponsorship and Pontiac picked it up.