This is only if you have a factory amp. For it to turn on, it must get 12V from the pink wire. Basically a remote turn on wire.
BLKGP, that was probably in response to stitch's question.
If it was a plastic harness theres a large possibility that was what it was for. GM seems to have made the HU an integral part of the HUD and the DIC which means for newer cars your options are stock with info, or aftermarket without info. There might be a solution, but I have yet to hear about it.