Has anyone tried or does anyone see why you can't replace the
factory door subwoofers with efficient 8" single voice coil subs -
Infinity's are 92db - by feeding them with only 1 pair of amp leads
and then sending the other pair of amp leads (rerouted at the amp
output) to a single 10" DVC mounted in a box in the trunk?

 

It will work, but the factory speakers are also lower impedence models. Say, you get 100 watts to the Onkyo 8's from Ford, it'd only be 25 watts or less to your Infinitys. Your actual best bet is to replace the amplifiers, too, but that is a PITA.

 

And they are oddball rated VCs at that! 1.2 ohms per coil. Put a 2 ohm speaker in there, and they will not put out as much power as stock. Put a 1 ohm speaker in there and you may (probably?) **** off the factory amp... I stuffed a pair of JL Audio 8W3V2s (a bit less than 2 ohms per coil) in my doors, driving both voice coils and I love it (search for a thread on 8W3V2s...) I would do what Infinity suggests and use a seperate amp to drive the rear sub. If you use a real sub, it will need a LOT more power than you are going to get from one channel of the factory amp.