Wiring amp to factory headunit?

My buddys car is an 02 Acura tl with navigation. I believe the navi uses the aux outputs on the back of the headunit. So I'm looking for opinions on the best way to wire the amp to the headunit without using the factory ouptuts. I know there's those devices (I can't remember what they're called cause I've never used them before) that splice into the front/rear speaker wire and have rca outputs on them. Has anyone used those before? Is the sound quality any good?

with my first setup i used them to wire a 4 channel amp off of a regular 2 rca cable. there was no difference in SQ. i think that your talkin about rca splitter (dont know if thats what they are called that just what i call them)

Yeah, I kinda feel like an a** cause right after I posted this I looked a few posts down and there's 3 posts about line out converters, which is what I was thinking of. Does anyone know if it makes a difference if I run the converter from the front or rear speakers? Also could someone recommend a good brand of converter?