Still digging around and finding all sorts of neat, and questionable, stuff.
I wondered why the ashtray wouldn't open, and found this bass control knob and a USB input cable blocking it.
Apparently, there was an amp and/or sub in the trunk that is now gone.
I confirmed that there ARE rear speakers, but they're not exactly in the stock mounting points. I tried to remove a dash grill and door grill and, even with the screws out, they were being stubborn, so I stopped before I broke anything.
I can see non-stock satellite tweeters in the dash, non stock coaxial speakers in the doors, and the frame of the rear speakers looks to be the same reddish frame color.
The head unit is ...well, WAS ... a nice Kenwood Bluetooth/Garmin GPS unit with a backup camera. That's probably what the USB port is for. Unfortunately, it's in protect mode with a "DC offset" error. Probably because all of the stuff in the trunk area is now gone.
At this point, I'm thinking I'll keep whatever speakers are in there and replace the head unit. I'll talk to the stereo shop about options. I really don't want a trunk full of amps and subs, though.
Oh, and I started to remove the shifter boot for a closer look, but there are some extra screw in there, audio wires ran down the tunnel, etc. I called it a night. I'm fairly certain it's not stock, though.