Hi, love this site, so much great information and great people here!
I’ve had only a couple of systems that I have put together over the years, but have tried to make them count.
The first was in a Fiero 30 years ago, used MB Quart 4”, and JBL titanium tweeters in the dashboard locations, and a pair of 8” woofers in small boxes, downward, up under the dashboard. The highs reflected off the windshield, and aimed to provide a nice soundstage. A 30w amp was placed behind the passenger seat.
I now have a 2009 Corolla with the same JBL Ti tweeters in the factory locations, a pair of Infinity 6.5 in the door locations, using handmade JBL 4406 studio monitor crossovers (air core inductors, nice capacitors).
The doors use some 1” closed cell acoustic foam panels behind the drivers, and a few small rolls of bubble wrap to help kill the back wave. Sealed up using 1/4” plastic panels, screws, and butyl tape/deadener.
The amp is an old Nakamichi PA300 I picked up off Craigslist and replaced a bunch of the parts inside, biased into class A, with fan added. A KENWOOD DMX706S head unit replaced the oem one (hallelujah!) I had used with a loc for a few years.