@ bavArian, greetings first to Bavaria!

(If I take that from the nickname correctly ...)
The first thing I would be interested in is which amplifier you were thinking of. I hadn't found a small four-channel with DSP. I already had a Mosconi Pico II on the screen, but even this one uses too much power. My voltage converter only manages 80W continuous load, 130W peak.
I would also like to use the two-way front, but I don't like the tweeters on the Dashrail. These are too obvious there. They don't stand out so much above the windscreen, I think.
You write, "a small kickwoofer in a closed box under the A-pillar". Uh, did you take off the footwell panel? I can only reach a construction height of 4.5 cm there. Since I'm already on the sheet metal. If I now screw an MDF multiplex board onto it, I might reach 6.5 cm. I have to look for an extremely flat speaker set. In my view, there is no way of thinking of a housing where you could still install it. I rather think that you even have to find a loudspeaker here that has a very hard suspension so that it is not damaged in "free-air mode". I just looked at the Peerless, but I have too little idea about it to rate it. So I have to read further and acquire further knowledge.
50-60Hz is "actually" not enough for me. As a fan of trance and techno, you also want to feel lower frequencies.
Fortunately, as far as the generator is concerned, I already have an alternator ... with 78 amps. Unfortunately, my previous owner did not make a 12V conversion, so I now have to live with 6V. Current without end, but voltage, ... well.
But I was just outside again ... Because my rear side panel is currently removed, I could see that there is another 20x30cm space under the window crank under the window, installation depth about 9cm. There would be room for an 8 "woofer. But again not without problems. Again, it is not easy to build a housing. Free air would be better here. And, ... even if you don't drive such cars in the rain , but at least when washing the car, all the water that runs down the window drips onto the speaker, not ideal.
Is there actually a window seal? I think I'm going to open a new topic.