Installing a sub on a Monsoon sytem

I am installing a sub and amp in my 02 TA. It has the Monsson system in it. I tied a high level converter off the back 2 sub speakers, and I am getting movement on the sub when I turn the volume down. It also pops a bit when the amp turns on. Any body have any advice for me?

My amp has high level inputs on it and I just tapped into the 4in rear speakers and it worked for me, but any cheap radio with true rca,s will sound better all around. Much better...