Well, if the crossover does what you think it does, that sounds correct. Now, with that said, most stock systems put out around 15-20W RMS. Now you said you were good with a "rink dink 25 watt" line, however, that was only powering the sub. If you have 25W trying to power a speaker and a sub, you're not going to get much.
You'd be much better off picking up a cheapo powered sub. They
start at $100 and go up from there. Here is
all of them. If I were going to get one, personally, I'd get
this one. Most of these are going to be fine on a 10 gauge power wire which is completely trivial to run to the back. Of course, no compact system will give you the sound of a full sized system, but from what you said one of these small ones would work nicely. Both of the ones I linked have speaker level inputs.
With all that said, no matter which way you go the speakers are not easy access in the Mirage in terms of wiring. So, it will be a pain without an aftermarket headunit. All 4 speakers are in the doors and it's a tight fit. I originally wanted to run new speaker wires from my amp but couldn't make the wire fit. If I couldn't run wire to the speakers it's reasonable to say you couldn't run it from the speakers. If you do some tapping in the harness inside the car that might work, though. But it'd be far easier to pick up an aftermarket head unit.
So, to install any external speaker, be it a new one you buy or the ones you have, you will have to take the stereo out of the dash and splice into the wires either way. You'll have to run the wires back to the cargo area. If you were using an unpowered sub to avoid work, that ship sailed. You're looking at equal work for setting up a powered vs unpowered sub in the Mirage.