Hello alfonso_ordo,
Here's a link to the wiring color code for the
2004 Silverado. It appears that there is not a 12V+ switched lead in the factory wiring.
Anyway, you really should not take the power and ground directly from the OEM wiring. The wiring itself is not big enough to provide the current that will be drawn by an external amp. And the fuse will not support the additional current either. You'll end up with blown fuses and burnt wiring at a minimum, and perhaps other damage to the vehicle electrical system.
It's best to power external amps directly from the battery and ground them as close to the amps as possible. Here's a link to an excellent amp installation video:
http://www.crutchfield.com/Learn/learningcenter/car/amplifiers_movie.html
Crutchfield also provides complete installation assistance, and lifetime technical support when you purchase from them. If you shop their "specials", their prices will be competitive.
If you are a first-time purchaser at Crutchfield you can get a $20 discount off a $200 order if you use a "friend referral code" such as mine, which is: p7sdj-hrx8q-9800g, plus free shipping. Oh!, I get a $20 merchandise credit too.
Hope this helps.
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