I would say nix the idea of a 460 amp trying to push a 10" sub. Don't get me wrong, it could work, but it won't perform. Reason being lack of power for one. What I mean is you have a convertable. Lots of outside noise to compete with... esp. with the top down. You just won't hear it let alone feel it. Reason two is design. The 460 amps were designed to drive the small crap woofers at a specific frequency that just won't do a 10" sub justice at all. The frequecy response is just too narrow.
It sounds to me you would like to keep the bass inside the cabin area and stay away from putting a sub or subs in the trunk. Am I right? I hope so. Subs in the trunk of a vert does work.... with lots and lots of power. Then you need subs to handle lots and lots of power. There goes your trunk.
Heres what I would suggest. Since your getting good sound from your Pioneer headunit driving the Alpines in the doors, I say step it up by keeping it simple. Lose the rear speaker enclosures in the side panels.
Now, now... hear me out... I would go with a small sealed enclosure on each side replacing the factory plastic pieces. This can be done with wood and a little shaping to make it fit or go full on with fiberglass. Any competant installer could make this work. I would then choose a GOOD shallow 8" woofer to go in said enclosures. I would go with a JL Audio 8W-0 or a Kicker Comp. Anything such as a JL 8W3 will be too deep to mount, so we want to keep it shallow. Now, find a good 4 channel amplifier of your choosing that puts out a CLEAN 100 Watts of power per channel, and has an adjustable crossover. Hopefully your Pioneer has a sub output or at least front AND rear RCA outputs. Use the front channels to drive the Alpines with the crossover set for HIGHPASS at around 60-80 hertz. Then use the rear OR sub output from the Pioneer to the amp to drive the 8 inchers. Crossover set to lowpass at the same frequency. You will be very suprised at the sound a good pair of 8s will provide as long as the enclosure is good and the crossovers are set right. If anyone tells you that 8s won't fit as I described, then turn and walk away. I've done this install in many verts, Mustang or not, in 2 different states without complaint. The key is keeping it simple. Headunit-- amplifier---2 door speakers---2 small subs. Simple. You could even add some nice tweeters in the factory location driven off the Pioneer for a little added "shimmer" to the sound. But I think the biggest difference will be the added power to the Alpines in your doors. With a well set crossover and CLEAN power, they will SING!
good luck
Mach 460 amp to power aftermarket subwoofer? 
