Back about 13 years ago (when I was in to all that stuff), I installed systems like this into 3 Contemporarys and 1 Shell Valley. The person who was building them in the Denver area had to have sounds systems for his customers.
Some of the tricks to making the systems work well are:
1. Use separates all the way around (i.e. 5 or 6 inch midrange with separate tweeter)
2. Now days, companies like Phoenix Gold make a good 5 channel amp that provides good power and clarity (instead of using two separate amps). An amp is not designed to make the music louder, it is designed to make it clearer as it gets louder. Most often people under power their speakers and that can lead to distortion and damage.
3. I always hid the main speakers the best I could. Perforrate the carpeting and install the front mid range speakers in the sides of the footboxes (make sure the back of the speaker can be sealed into a small compartment, away from the header heat or the open air). The remote tweeters are less than 1" wide and can be placed someplace higher. I usually embedded them into the corners of the door (just inside the top hinge) because they are really not noticeable and that give you great sound.
4. The rear mid speakers were cut into the trunk wall behind the seats and then covered with the perforated black carpet so no one knew they were there. The rear tweeters were placed at each corner by the softtop ferrels.
5. For some bass, the sub does not have to be shown (the sound is nondirectional). There are areas in the trunks of most Cobras (above the rear suspension or behind each tail light) that provides you the space for two 8" subs in a custom box (about 1.8 cu.ft). Again, by cutting a 5" hole (not for the speakers but for the sound travel through) in the trunk wall behind the seats, it allows the sounds to pass through without seeing it (perforrate the carpet that covers the hole). The bass will actually use the trunk to make the sound waves longer (more bass) so 8" subs are more than enough when powered well.
5. Although I never had the chance to use one of the hidden systems that are produced today, I used the glove compartment for the main unit and it worked well by deepening the box.
6. The amp or amps (depnding home many you want
) and any optional voltage stiffeners are great in the trunk.
7. For an antenna, I simply used the wires from a stock one and remove the metal antenna part, then attached it to the bolts of the window support. Most of the time this worked fine depending on how the car is grounded. You may want to go more high tech and use the new hidden antennas.
These systems were about 500 watts and $1,600+ in "quality" parts including a high output alternator. The sound is great at even highway speeds because it is clear, though loud.
Easy to do yourself with some help on custom speaker enclosure designs that can be found in many places on the Internet.
Sorry for lengthy opinion.