[clev]

Does anyone know if there is a direct quality subwoofer that will replace the one behind the rear seat?

 

[RightWingNutJob]

I don't believe anybody makes a drop-in replacement for the powered OEM sub-woofer on your KR.
There are aftermarket options that will fit behind the rear seat - but they aren't just plug and play.

I run a Supercrew Sound Box, with two 10" subwoofers. It runs on a separate amp with a crossover network.

Ford F-250 | F-350 Subwoofer Boxes and Dual Subwoofer Enclosure

 

[clev]

Thanks, I'm check out your link.