Bose System in 2003 Yukon

I am trying to hook up an amp in my Yukon and take only the door speakers and rear speakers off the Bose amp. I am leaving the tweeters and center console sub on the Bose. I located the speaker wires behind the Bose amp, cut one set at a time and hooked them to my after market amp and got no sound. Is there a secret to this? I know I am not the only one having this porblem. HELP SOMEONE.

B