You will need an amp and an ohm load meter. Put in fj cruiser speaker in ebay you will be amazed at the available options. One is a site vender I'm sure someone will add to my post. Its a great system two 10" and box with JL stuff.
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Ideally you'd run RCA cables from your sub output on your head unit to get signal back to a new amp and subwoofer. The stock subwoofer's amp only has enough power for itself and really shouldn't be split. If you want more bass and want to save money, you can search for upgrading the stock amp and sub while keeping everything in the stock housing.
Same thing happened to me. My friend gave me a sub+amp. You won't be able to power the new sub with the stock amp.you would have to get a new amp to power the new sub and you need to run a new power and negative cable to it. If you have the fjammer head unit you will not be able to run RCA cables from it due to the fjammer not having any. So you have 2 options, get new head unit or you will have to get an adapter that ties into the original wires that go to the factory sub, it costs about $20 to do that and I'll give you the name of the adaptor if you need it