This is what I use (or something like it, just a representative example) on our big van and older SUV's:
https://www.amazon.com/CIPA-60200-Wide-Angle-Lens/dp/B000TCI7K4/
I like them better than cameras because they also help with seeing blind spots while driving. (in addition to making the area below the window visible when backing up) Cheap and easy to install.
And older cars? LOL, I'm right there with you... we have five drivers in our family with a Ford E350, Chevy Tahoe, and a Buick Rendezvous (remember those? The Tiger Woods car...). The other two have backup cameras.