I did a dumb thing.
Having followed this thread since I got a used Spark almost a year ago, I have been on the verge of doing some these suggested upgrades many times, particularly after a long drive on a sunny day. Cruising with the windows down, but, unfortunately, the volume down, too, due to aforementioned issues. However, I never pulled the trigger due to the lack of a very simple plug and play option that would require minimal work. I even bought the Mazda cable adapters at one point, but hadn't use them yet.
A few months ago, I bought a new home theater receiver to replace the following cheap RCA I had gotten years ago:
https://www.amazon.com/RCA-5-1-Channel- ... 000P6J79E/
It was cheap, but I liked it a lot, particularly the subwoofer. It had been sitting in my garage until I finally got rid of it today, but I couldn't part with the subwoofer and decided to keep it around. As I moved it around, I noticed on the back label that it had an impedance of 8 ohms and I was reminded of this thread and the fact that the front speakers were oddly (for a car) the same.
Why not give it shot? This sub is not powered and just has the two bare wires for use on the receiver. I disconnected both rear speakers and hooked this up to one of the leads (using the adapter), not expecting much. To my surprise, it worked like a charm, and doesn't seem to have caused any issues (yet). I set the fade slightly to the rear channels and I'm bumpin California Love no problem, vibrating my rear view mirror (in a good way).
Two things: I don't know if this will eventually cause electrical issues, and this thing is LARGE. I basically gave up one-third of my trunk space, but now it's full of bass.