Seating Arrangements for The Brady Bunch?

Something has been plaguing my mind lately... How the hell did the Brady Bunch fit nine friggin’ people into their station wagon for that cross-country trip they took to the Grand Canyon? 

Granted, I haven’t seen the episode in question in several years, but if memory serves, I believe it was Mike, Cindy and Carol in the front seat… but then it gets fuzzy. How did they get six more people in that damn car? A coworker just told me that back in the 70s, station wagons used to have TWO back seats, in addition to that cargo area in the back. But I found the above ad for a Brady Bunch-era station wagon that's remarkably similar to the one used in the show and it has no such "jump" seat. Even if it did, it still seems like it would be awfully crowded for nine people (plus their luggage and camping equipment) on the long journey from California to Arizona. Maybe they got out a lot to stretch their legs... They also probably got a nice break when they stopped in that ghost town to hang out with Jim Backus, but still…