You could install almost any reasonable size speaker, including subwoofers,
if you put them under the rear seat.  An extra advanatage is that they can
be in wooden or other enclosures and thus sit right on the floor,
simplifying mounting and ease of removal.

You can either cut proper size holes in the facia plate that covers the
space under the seat, or use that as a template to make an additional one
with speaker holes, and thus still have the uncut original.  Earlier years
have fiberboard-type OE facias, while after about 1986 they are made from
plywood.

Budd Premack
86 Syncro, 73 Super
Minneapolis, MN (Land of Sky Blue Waters)