The first thing I did when I got the wiring harness (Metra 70-2105), was realize that there were not enough wires and after I found a wiring diagram for my car they were in the wrong places even. so I started by ordering an additional wiring harness and began pulling wires from it to add to the first one I purchased.
To pull the pins the first thing you need to do remove the white plastic pin on the side of the harness. Be careful when removing this because the top of it will break off and you will not be able to remove it. I recommend getting under two sides of it and pulling evenly rather than prying it out.
Once the Long white plastic pin is removed you can remove both the white and blue clips that hold the pins. Be extremely careful from here as the pins break easily and this is why I ended up buying 3 wire harnesses.
After you get the white and blue plastic wire clips out remove the pinned wires by using a small flat head screwdriver or something similar to push back the plastic clip that is holding the wire in and pull firmly but evenly on the back of the wire and it should slide right out
When you get all the wires removed, determine which ones you will be using and mark them so that you know what each wire is going to be used for. I tried to keep the extra wires I added to the original wiring harness somewhat similar in color to the ones coming from the back of the radio I was installing.
When you start putting the wire back in to the wire harness you can either put the plastic clips back into the harness first or put the wires in the plastic clip then put the clip in the harness.. I used the latter to make sure the wires went in as far as they needed to. Just make sure when you put the clips back in the harness that you have the right ones on top and bottom since the clips can go in either way and I would still recommend being careful with them and don't try to force it if it feels like it is not going to go. I used my flat head to widen the hole just a tiny bit so some of them would slide in a little easier. As long as the wire sits in the clip the way they did when you removed them they are fine.
The way I wired it allows the factory amp and the sub to work.. incredibly well I might add. The new radio is 10x louder than the factory one. The new radio does not have XM in it so I can't say if it still works with that wiring. Also, I'm not sure if Onstar works either since I never use it.