That is why you currently have two connectors. The Onstar connection uses the existing black plug, but the VSS plug is the brown one and will be the 3rd plug used with the Nav unit.
The Nav unit will have four plug-ins and all will be used if you have rear DVD system. If you do not have the Rear DVD, three will be used, The Brown, Black, and Purple. THe Onstar connection plugs into the Onstar box and the two loose wires (Red and Black) pin into the existing Black plug. The black plug should have two available pin slots. It's very simple. I purchased a nav unit and did the install just a few weeks ago and used the harnesses listed above. You can purchase individual wires/pins and create your own connections, but I elected to spend the money and purchase the already made harnesses.