You may wish to consider purchasing a wiring harness that will allow you to plug directly into the vehicle harness without tapping or splicing. Here's a link to one such item: This one appears to be for your vehicle.
Installer.com shows the following color code for the radio and speaker wiring for the 2007 GMC Sierra.
Red: +12volt Acc Switched
Yellow: +12volt Battery Constant
Blue/White: +12v Remote Output (Amp Turn On)
Red/White: Reverse Light Trigger
Yellow/Black: Parking Break Trigger
Orange/White: Auto Illumination
Blue: Powered Antenna
Brown: Phone Mute (optional)
I would recommend that you verify the speaker wires before doing any splicing.
Attach a speaker to a pair of test probes and leaving the harness in and working, probe the back of each pair as coded above to insure that they really are the correct wires. Most likely you will want to convert the rears to low-level, so start with those. Probe the Violet(+)/Violet Black(-) pair and you should get sound in your test speaker. Likewise, do the Green(+)/Green Black(-) pair.
The only other wire you would need from the stock radio for an external amp would be the blue/white turn-on lead and if your LOC has audio sensing turn-on, you may not even need that. Bring power to your amp directly from the battery (fused according to amp manufacturer recommendations) and find a ground within 3 ft or so of where you intend to mount the amp.
Hope this helps.