So first off I just wanna say this is the first time I've ever put a system in a car, so take it easy on me. So this afternoon I was working on getting my alpine mrv f450 amp and a jl sub I found(I know for a fact it works) in my 03 corolla, last week I put in my pioneer fh-x721-bt or 720 and it all works well except the sub, the amp turns on the deck works but no matter where I put the rca in the amp it just doesn't work, and I've tried changing around all the outputs to the sub but nothing. The blue light on the amp turns on so that's where I'm confused, I tested the rca with a headphone to rca cable, also doesn't work. I don't know what else to do, I've tried almost everything except for the really time consuming stuff. If anybodys wondering I had the amp in 3+4 trying to bridge it and I tried switching it to just the 5 channel also didn't work. I don't know what else to do im incredibly appreciative of any suggestions or help anyone can offer. Thanks in advance.