Ideas

For full transparency I have a 2011 4Runner when I bought it the previous owner had wiring left for a pick up sub under the passenger seat, within the first couple of days I realized all the speakers were blown or weren’t playing so I replaced all the door speakers and ran them to there own channels on a 4 channel crunch amp located under driver seat. I used the wiring left from previous owner for a pick up sub I got off of Amazon and it hits but I’m looking for more, to my knowledge the factory Jbl sub in the back dosent play I don’t know if it’s blown and I don’t believe I can connect it to the pick up sub as it has a enclosed amp. The question I’m asking is should I upgrade the pickup sub to something with better reviews? should I try and hook up the two subs to run together ? Get a 5 channel amp and go for a shallow mount sub. I’m not trying to go with a crazy music setup as I daily this thing and use almost all of my cargo space. Any information or help would be appreciated