that box the best buy person was talking about is the relay box.
for a while(i don't know the exact years, or models it includes) chevy has decided it would be a brilliant idea to put all the door chime, seat belt warnings, and other chime features into the head unit. so if you yank the hu, you loose those features--personally, it would bother me, but not $200 worth of botherment... i would install a diff hu, and deal w/o the chimes. the module he was talking about keeps these features--and is not cheap.
there are plenty of aux input adapters that can plug into a headphone jack.
the best would be a hardwired model, though with the truck being a gm you would also need a gm-to standard, and standard to gm antenna adapter..
i have used a model like this in the past-- it works, but no better then any other wireless model i have tried..
this is the hardwired version. this will work better becuase it disables the radio antenna while the box is on, but the trouble is that your working on a gm truck-- with the mini-motorola style connectors. this uses standard sized motorola connectors.