Remember that under the dash is the standard location, I just used larger speakers which required the use of spacers.
I don't know if rear facing speakers on there own would be very effective, I have four and with fader the set 50 / 50 the front speakers are definitely the ones that give most of the sound.
Alternatively you could get the housings that fit them at the rear of the tub, I personally didn't go for this solution as I thought I might damage them when carrying rubbish in the back.
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