Cooper3824

Hi everyone, I am not that familiar with car audio, but I have a nice amp which I want to add to the current Bose system, maybe in the future I will replace it, but not just now. I simply want to add the amp and some speakers for under the rear strut bar. Leaving the Bose system working as is, where is the best place to tap off the sound signal? Leaving the head unit or where it is attaching to the Bose amplifier? I assume this is a line level signal, is that correct? Or can I best tap off from the speaker wires and use a high level converter to go in my amplifier. I read that it the is best to tap off from the front speakers, due to the audio curve for the rears. Any help will be appreciated! Thanks, Tom