So what do noncamper owners do about subwoofer placement? I'm
probably going to fit my 10" Alpine sub into a particleboard box and
stuff it under one of the rear seat rows, using bungees to stop it
sliding out from underneath.

The midrange speakers I'll probably mount under the dash if
possible... doors are really an unfavorable location due to damp and
shock, as well as wiring fatiguing.

10-disc CD-changer... dunno, maybe under a back seat as well.

Headunit I'd like hidden, but it must be line-of-sight for the
driver. Maybe in a dashtop console with a door to hide it.

If all components are out of sight it'll look as though there's no
stereo, reducing likelihood of breakins substantially.

If I had a high-roof Hiace I'd probably build a false ceiling over
the cab and install everything but the sub there.