On the Crutchfield support site, I found an audio wiring diagram
that gives the amp location as being "to the right of the glovebox" ... and, sure enough, it's in the sidewall next to the glovebox.
So, I'll see if I can remove the glovebox and the plastic panels and see if anything seems odd with it. I do have a dealer visit scheduled next week (also going to have them do the service on the fuel-filling-pipe recall), but I'd like to get the radio working on my own before then if possible.
I'd still be open to any words of advice.