my sister has a 98 sierra and i managed to put an older kicker cvr behind there. it was only a 10 and it was in a ... dam i cant remember the cuf of the box but it was made out of half inch mdf and basically took up 2/3rd's or the space behind both seats.
almost any sub that is under 5 inches mounting depth you will have almost no trouble getting it into tht small space. its all about the planning, and drawings help alot.
blaupunkt, pioneer, kole audio, um and i cant remember who else makes flat subs.

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