I have 2 sets of preamp outputs on my hu. What do they do . I have never put amps in my car and thats what I am assuming they are for.

 

You are correct. They are low level signal outputs for an external amp. They do not pass through the decks amplifier section so they don't pick up the added distortion the cheap internal amp produces.

 

They should also be labeled front and rear... some have 3 sets, the 3rd being a dedicated subwoofer output, which is generally non-fading...meaning that when you use the decks fader it fades the front an rear signals but not the sub.