hissing sound with new line out converter

Hi, I have a 2004 Toyota Camry XLE with the JBL premium sound system. I had an alpine headunit in it with component speakers and JL subs but I broke my headunit the other day so I put my stock toyota headunit back in with a line output converter after the factory amp to connect the front component speakers to my JL amp for the front speakers. You have to catch the speaker wires after the amp because the volume is controlled by the factory amp but there is a constant hissing sound that is unacceptable. I know this is a problem sometimes but iv'e never had this problem before so how exactly do I eliminate this hissing and static? Thanks a lot.