I have to agree with placing a 3" in the dash, we're lucky to have enough room in our cars to fit one in there. I have a illusion audio c3cx point source in there and it sounds amazing and makes the sound stage much more believable with having 350hz and up in the dash with a lot less blockages between them and your ears. Here is what I'm running in my car:
pioneer avh-4200nex
Mosconi 4to6dsp
Arc audio xdiv2 1100.5
oncore 10" sub (new company from the original guys of image dynamics)
Illusion Audio C6 midbass
Illusion Audio c3cx point source
stock speakers for rear seats (i fade 95% front anyway)
here is my stack fab enclosure for the sub, looks 100% oem: