installing amp and woofers 300 signature series
 

I am wanting to install my 2 amps and woofer in my new 2008 signature series but I need help. Where or what fuse can I tie in the trigger wire for the amps? I plan on tieing in to the wires on the existing rear speakers for one of the amps and using a step down instead of running rca cables to the back of the radio for both because to be honest I dont know if there is rca outputs??? I plan on piggy backing two step downs for both amps and the woofers. Does anyone have any better ideas? My main question is the trigger wire and where to tie in. Has anyone had problems with the internal built in amp causing problems with these additions??