How do I hook up an amp to stock stereo in '07

[olskoolchevy]

I traded in my 07 Silverado Classic for an 07 GMC(newer body style). I pulled my amp and subs out and want to put them in the GMC. When I had them hooked up in the Chevy they put in something (or that's what they told me anyways) that allowed me to hook up an amp to the stock stereo. I'm sure the GMC doesn't have pre-outs so what do I need to hook the amp up and how much do they run? Thanks in advance.