first - you won't be heard a mile away, so just forget about that requirement. also, avoid the plexi bandpass boxes, they are for looks, not performance. build a ported box for those subs - to the mfr specs. or buy one of the boxes in Murph's Classifieds thread.

Second - you lose your fader with a two channel amp powering front and rear speakers. buy a 4 channel amp since they are more flexible anyway. just pick one in your price range.

third - tweeters produce high frequencies. their location is important. i don't recommend using coaxial speakers (2-way or 3-way) in those doors. buy a component set - it will come with separate woofers and tweeters and a passive crossover. install the new component tweeters in the factory tweeter locations. some minor modifications may be required - get used to making modifications. also, remove your door panel and verify how much mounting depth you actually have - and make sure the new speakers are not too deep.

fourth - you don't need a cap, it won't help. you need good power wire - 4 awg minimum, and you need to do the big 3 (engine wiring upgrade). you won't have enough power to warrant a new alt.

last - you can pay a professional installer to get a trunk like that. if you want to build it yourself, it can be done. but it requires a high level of fabrication skills, patience, and a good work shop.

just because there is plexi does not mean it's a bandpass box. a sealed or ported box can have the back made out of plexi instead of wood. if you insist on having plexi that is visible from the trunk you can make a custom box with subs firing forward (through rear seat opening) and make the back of the box out of thick plexi. note that the material required for the fabrication will cost more than your speakers and subs combined.