I'd be willing to bet there is an aux capability on the head end, just that no jack is exposed to the user. Car A/V places could add a jack somewhere. I've done it when I replaced the head end in my old Toyota.

Intriguing idea. You may well be right -- the high-end Limited does claim to have an Aux input, so it's quite plausible, although it wouldn't surprise me if there are software implications as well. (That is, something has to tell the Audio UI that that jack exists.) Worth thinking about: thanks for the suggestion!