I've never been happy with my radio and speaker system, installed by the previous owner. A Phillips AM/FM/Tape with two front speakers.
I've said in the past, "I tend to be a purist, but I'm not a masochist." I'm also not an audiophile, so I decided to install a modern Retro-style radio system (CASMFG USA-4DIN driving four 40W JVC 4" speakers).
I have and like my Kinder Seat, so parcel shelf speakers are not an option. My solution for rear speakers is shown in the photos. The first photo was taken before the passenger side pilot holes were enlarged to be like the driver's side. The speaker spacing provides for the Kinder Seat and its passenger. I'll re-glue the carpeting to the wall everywhere except where the grill holes are. There are already lots of holes between the cabin and the trunk so a few more small ones should be OK. Structurally, the bulkhead will provide the same backing for the carpet. I've tested the carpet effect on sound by isolating one of my family car speakers and pressing the carpet tightly over it. The effect is a minor reduction in treble and a slight reduction in volume, an effect that I find acceptable for rear speakers.
The speakers are accessible because the speaker boxes are hinged and latched wooden $2 craft boxes from the dollar store, covered with felt and lined inside with thicker jute. There are convection air circulation holes in the top and bottom to provide any cooling required.
My front speakers will be in the panels above the driver's and passenger's legs, so all speakers will be out of sight.
The only difficulty was designing a grill pattern (I found a pattern on the web), then drilling (142) 1/16" pilot holes, then re-drilling them to 3/16" and spacing the speakers away from the rear of the wall so as not to have speaker contact with the wall except where the mounting flanges are. Normally, a four inch hole is required for speaker mounting - not acceptable for me.
Since I know nothing about sound systems, I took a mechanical engineer's approach. I hope when I turn up the sound, I don't vibrate the taillamps off the car.
Tom Kizer