The factory (left rear bulkhead), non-Bose sub is 8" and sounds a bit worn out. Crutchfield doesn't list anything "over the counter" but I know there's got to be something I can do. Either replace the stock 8" with a new one (not factory), or figure out an enclosure that would let me squeeze a 10" in the factory space.
FWIW, stereos are at the bottom of my list of things to upgrade, so I'm not thinking of anything fancy. But between the truck needing TLC and the boat needing an upgrade, I figure I've got to look into this stuff.