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if it's a 2004 and has a factory sub than there is probably a stock amp driving every speaker, you can pick up a harness that will let you utilize the stock amp with an aftermarket radio, if you run an aftermarket amp you MUST bypass the stock amp or you will be blowing up some electronics.

if you want to use the stock radio you'll need to use a line converter (speaker to rca's) after the stock amp, and use a quality one.

there are a few mfg's for the "stealth" boxes...MTX thunderforms and JL stealthbox, you can order the mtx without a sub but the parameters may be off to what your friend has.

is it a crew cab or the 4-door 2 seater?
if it has the rear seats your options are pretty good and you do have a little room to play around.....2 seater though and he'll be limited.........