hello, does anyone know if a jl w3 sub fit in the stock rockford fosgate enclosure in the trunk? thanks.

 

A few things to consider, open the box and take out the stock sub measure it, then go to jl's site and see the specs.

Also, check for the ohms, make sure they match and also sub should match the box volume for it to work at its peak, it might be a guessing game to determine the stock boxes internal volume, unless there is a method to measure.

The stock box is junk, you might want to consider an aftermarket, like wicked(sp) I think looks nice.