well, there are bits and pieces of correct information, so let's see if i can help sum it up.
to keep your factory head unit, you'll need what's called a line output converter (l.o.c.). any audio shop should have these in stock. even an electronics store like best buy. basically what it does is convert your 12 gauge wire into RCA signals. so...you will need one for each set that provides sound from your headunit. tap all wires
before the factory amp.
amp 1 = your 4 channel for the highs. it will be 6 channel if you want to do your trunk speakers also.
amp 2 = your mono class-d sub amp
(note: i have not verified these wires, but the source has a 90% accuracy with me. don't know the colors of the trunk wires though.)
head unit > tan(+)/gray(-) > l.o.c. > amp 1 > Left front speaker
head unit > lt grn(+)/dk grn(-) > l.o.c. > amp 1 > Right front speaker
head unit > brwn(+)/yellow(-) > l.o.c. > amp 1 > Left rear speaker
head unit > dk blue(+)/lt blue(-) > l.o.c. > amp 1 > Right rear speaker
head unit > l.o.c. > amp 1 > left trunk speaker
head unit > l.o.c. > amp 1 > right trunk speaker
head unit > l.o.c. > amp 2 > sub
easiest thing to do with the sub is the infinity basslink t. i've heard it in a silverado, and it sounds great for someone who's not competing in sound competitions. l.o.c. and amp are built in. more info
here.
hope that helps.