Hi all, I have an 08 Cabrio passion. I wanted to add a speaker in the back but keep the stock radio 10. I have read that the stock radio only has 2 outputs for speakers. It looks like I dont have tweeters, but I do see some wiring under my drivers seat that looks like Smart sub?? I cant hear any sound from it so I'm not sure if only the wiring exist. Can any let me know how to confirm a sub and if another speaker can be added?