It completely depends on what you expect of your stereo system on what you should do. If you want the features of a aftermarket deck but only want the stereo to sound slightly better than the stock deck just change the deck and buy that OS2. But, if you want it to sound a lot better ditch the OS2 and use that $105 on the rest of your system on speakers, amps, and installation. 
I agree with Rod on this one, With the OS2 it will sound only slightly better then stock.
But keep in mind if you dont use the OS2 you will lose the Onstar/door chime and you will have to find an acc power source under the dash for the deck, the factory stereo plug has no acc power it is controled through a data wire. Also if I remember right your truck should have a factory sub in the center console, if so you will also lose that without the OS2.
In my opinion you should start with the deck and OS2, so you dont lose the Onstar and other stuff.
Then if your not happy change the speakers and rewire around the factory amp. At this point you will need to ad an extra speaker under the dash somewhere for the Onstar, it will nolonger be able to use the stereo speakers to talk to you. The factory amp should be under the back of the center console. You can get the speaker wires there and run them up to the deck or run new wires from each speaker , if done right you should still be able keep the factory sub working.
The next step is to add some good amps and subs and rewire it again!! By now you should see why us poor little installers HATE the word BOSE !!!!!

Your best hope is offer Rod lots of beer and hookers to take care of it.

(sorry to volunteer you Rod but your closer than me

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