2005 adding steering wheel controls

Google "2005 sierra adding swc" or "2005 sierra adding steering wheel controls" lots of people have done it.

Note, you may wish to figure out if/how to get your bcm reprogrammed to support SWC's before doing this... The tech's at your dealdership may give you a dumb look (maybe because they don't know it can be done, or perhaps just because they don't want to do it) and/or techconnect may choose not to do it anymore. IDK if/how well swapping bcm's with a truck that does support swc's, but I would suggest if you try this, to find one with as similar an option package and configuration as your truck as possible, only adding swc.

I have done this in my 2005 suburban LS with no steering wheel controls. I bought an escalade steering wheel of the same year, and I changed the steering clock spring that has wires for the steering controls. I did not have the bcm though, what I did was use a pac-swi-rc interface that I bought from amazon.
Goodluck!