Did you ever purchase the speakers you mentioned in the message below
and install them with some enclosure?
Regards
Bill
From jifjif@gmail.com Fri Apr 27 18:36:02 2007
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Date: Wed, 8 Dec 2004 16:38:26 +1300
From: ~Jeff~ <jifjif@gmail.com>
To: MX5List <mx5list@mx5club.org.nz>
Subject: Re: Speaker enclosures
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Generally speaking you want to have an airtight baffle which isolates
the back and front of the speaker. Your tub should be OK for this,
better still if it doesn't contribute any funky resonance of it's own!
Stuffing it with polyfill and layering it with roofing goo will help
too, possibly OTT for headrest speakers tho

it's been about 10 yrs since I've twiddled with this kinda thing, back
then rec.audio.car FAQ made for an interesting, if nerdy, read.
hope that helps...
On Wed, 08 Dec 2004 14:00:36 +1300, Bill Rehm <bill@kase.co.nz> wrote:
[...]