feedback noise from audio
 

Added a four channel amp. those that have their amped their highs with the Bose system, did you connect the line output control before or after the Bose amp?

there is seriously a billion threads about exactly that... if you had searched you would have found that you can do both.. but most people do before... and i'm going to assume that quation is also related to your post title... You will get feedback if you use cheap equipment or don't hook it up right.. you will probably have more if you do before the amps because you would half to turn your amps up more unless you have a good LOC.

yup, what Wikked said.

Bad ground?

all i needed thanks : )

before the Bose amp, with a powered LoC / signal processor such as an Audiocontrol LC7, or DQL-8

Chances are you have a ground loop between your head unit and your amp, as good grounds are abundant in the trunk.