Can't use standard extension cables because they are not standard cables. Mini USB actually has 10 conductors in the plug, most of which are used here, so a standard 5-conductor USB extension won't work.
Also, it's REALLY easy to fish a cable down the center of the headliner in a Prius because it's a hatchback and thus has a very short roof. Less easy in a regular sedan like a Camry. In fact it's even easier if you have a sunroof because that gives you easy access to the headliner, inboard, away from the curtain airbags. That's how I ran the rear camera cable in my wife's Highlander, specifically to avoid the side airbags.