You wouldn't need to mess with these Kicker crossovers at all. They're JUST for the front tweeters. I'm not sure how adding subs works on a stock system but, I think you end up splicing into the wires going to the rear speakers and adapting them for the subs. Subthump does have what you need for all that. Honestly though, a new head unit has sub outputs already built into it and it handles the crossover so, it's practically plug n play. If you're not into what kits are available though then I don't know what else to tell you. Upgrading the speakers and adding a sub is doable and will probably sound ok but, it's far from ideal.
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