How deep is the 10" factory door sub? Was thinking about shallow 10" in the door with a good quality 4" in the tweeter location. What say you?

 

Oh yea, how big are the factory tweeter holes and depth?

 

there is 3" exactly at the most shallow point, you can get 3.75" with a 3/4 mdf spacer.
the problem is not that you cannot fit a sub, the problem is that a subwoofer in a door has no real enclosure to react with the sub.
you see, most subs are made to work in an enclosure because the air pressure in the box acts as a spring against the driver. without this the driver will over extend itself and self destruct if you push it anywhere near it's rated power. not only that it won't sound very good.
it would be like driving a car with no shocks installed.

what you seek is either a fiberglass enclosure in the door that can be sealed or you need an infinite baffle type sub. these subs have the mechanical spring if you like to call it that in the suspension of the sub so they don't need a box to work.
i am looking very closely at the JL audio Zr800 CW 8" midbass driver.

the other problem with a regular sub is you really need somthing that will play up to about 300 hz in order for the 3.5" speaker to be able to meet up where the midbass leaves off.
a subwoofer will not play above 150 hz usually because the cone would be too heavy, the suspention too loose and the voice coil has too high of inductance, it simply has too much impeadance at thoes frequencies to be of any use.

that is why a sub woofer is called a sub woofer because it plays the frequencies below thoes of a woofer or midbass driver.

just like a lot of things in car audio, just because you can fit a sub someplace doesnt necessarly mean you should.

 

Thanks for the input, I was looking at a sub like the pioneer tssw521. Specs show It playing north of 100hz, I can't see the white papers on my phone but I can only guess on the Q value. By any chance have you put a spectrum analyzer in the car to see how far the door speaker plays and if you could use an eq to force the issue?
Thanks