As said above, biggest problem is that an AE86 is NOISY
The previous owner of my car stripped the interior and sprayed that blue
sound deadening coating over everything, then put the interior back with
extra sound absorbing material under the carpets... still nowhere near as
quiet as any late model sedan. Reality... quality sound systems are wasted
in an AE86 due to the high ambient noise level, but having said that, I put
in a small system...
6-1/2" splits in the doors with the tweets up the top of the doors, sort of
inside and just below where the door mirrors mount. Passive crossovers
down beside the mids. Single 10" sub in a custom sealed box. Used two
small amps, one driving the splits off the front channel, and the other low
passed and driving the sub off the rear channel.
Sounded OK round town, but couldn't compete with road noise at highway
speeds. Haven't used it since one of the amps died, and don't miss it