Americans age 40+ -- Of course you called the part of the station wagon behind the back seat the "way back," right? And you probably remember the "way back" machine that Mr. Peabody and Sherman used to travel through time during Rocky and Bullwinkle, right? Which came first? I'm thinking some cartoonist heard his kids talking about sitting in the way back, and thought "What a great name for a time machine."
I am guilty as charged... an american, aged 40+.
We just called the back of the staion wagon "the back".
As far as the "way-back machine", my assumption was always just that it was their "way back" in time? since without it, they couldn't have found their way back.