Why do you want something that is not a component? A component will sound 10x better than a coaxial.
And like I said before, no matter what speaker you have, you probably arent going to get the speaker to make 30hz sounds. Even if the speaker is physically capable to do it, your door is not a proper enclosure. I'm not sure why you want to go so low anyway if you are getting subwoofer. Also, if at all possible keep the door speakers from producing anything under 80hz. Without an amp, you are running on very little power. I dont know what the stock wattage is but I would bet its between 15-20w. It should run ok like this on higher frequencies, but if you are trying to get any bass out of a speaker running on 15 watts its going to distort very very quickly. (I'm running 250w per speaker, and I could use more)
You can get high pass crossovers on ebay for really cheap that should do the job. If you are amplifying the speakers the amplifier should have that option as well.
With those subwoofers do you have any plans for a box? Even though its a slim design, it still is a 12" sub and requires a decent amount of enclosure space. If you are planning to stick it behind the seat keep in mind that with any box with more than 0.50 sq. ft. you are going to lose reclining room. You need double that for that subwoofer. Not to mention if you arent going to amp the regular speakers you wont even be able to hear them over the subwoofer. My best recommendation here is to buy this guys (
http://mr2oc.com/showthread.php?p=1647295#post1647295) MR2 Specific subs he is selling. Dont be put off by a sub only because its an 8". MR2's arent big cars and you could probably throw 175 at those. Even one of them should make plenty of bass. Trust me when I say it will make enough bass to completely cover the main speakers if you wanted too.
I'm not trying to tell you what to do here, but I have learned from my mistakes and would like to keep others from making those.
I wouldnt worry about mounting depth. If it doesnt fit make a spacer out of wood. Simple as that.
Kosan: I'd actually be happy to do it if you really wanted me too. I'm not pro by any means but I know my way around.