You really do need to bypass the amp. The cable is only like 15 bucks or so. If you don't want to order the cable you can do it by hand but it is a pain in the ass. I never let anyone install stereo equipement in my car. All the installers i have met are idiots. Most of them are highschool kids that don't even get trained on how to do it. They all do half-assed jobs in my opinion. If you need help installing anything just email me. I have installed a new head unit, amp, sub, tweeters, and new speakers in my car(it sounds bad ass). Let me know if you need help.
