I'm assuming the expensive wiring harness that you're talking about is to replace a Class 2 databus from GM. The pdf link from Nick is showing that you shouldn't need one so you should be able to get a harness for under $20 easily. GM started using the databus around 2000 on truck and 2003 on cars if my memory is correct. It would be easy to find out if you need to the expensive one or not by unplugging the stereo. If the harness matches the pdf from above its not, or if your door chime no longer works it is a Class 2 databus. A deck can be installed without the databuss module if it does 'need' one, but I really don't recommend it for ease of install and safty. The Class 2 databus interacts with airbags in a way that is more complex then I will ever understand.
Sounds like you need to do an amp bypass. I didn't think GM used an external amp on non Bose systems but hey I don't know everything. Does it have a sub maybe?
If there is an amp either bypass it, or wire the remote turn on (blue/white) wire coming out of the new deck into the remote turn on for the amp I believe it's the pink wire, but your new harness should map that out for you. This will turn the factory amp on. If you have a volt meter you can check the resistance of the wire in question of being the remote turn on to the amp and you should get a really low odd resistance level. If that doesn't work the bypass will, which you'll need a volt meter for as well If you're doing it without some sort of schematics and even then I'd test every wire before connecting something to it. You may want to do the bypass either way sometimes those factory amps really mess with the clairity of the music.
If you wire it and it kills your trucks electrical system I'm not responsible, it's sad I feel like I even have to put that in there.