Hello, I need help wiring a Line out converter to my factory head unit.?

I have a 2008 Silverado LTZ with the factory GPS navigation head unit and am going to hook an amp up to it. I know i need a line out converter. Most people say to wire it to the rear speakers but I want to wire it right behind the head unit itself incase i want to upgrade later on. does anyone know exactly what wires are the L %26amp; R (+) and L %26amp; R (-)? or a diagram of the wires? thanks in advance!Hello, I need help wiring a Line out converter to my factory head unit.?
Left rear speaker- brown (positive) - yellow (negative)

Right rear speaker- dark blue (positive - light blue (negative) Hope this helps. Good Luck!

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