First, I'd like to credit user "jteske88" for the idea and especially for this first image. My equipment had no labeling anywhere for the pinout(s). Thank you!
That did come from a similar thread that had a splice in the trunk. Who wants to run a wire that far if you don't have to?
You'll want a few things for splicing into the harness, like a knife and some tape. I'd very much suggest soldering these connections, but I did not. I twisted and taped like a nub until I go back in there to do it right ..and it is only for a signal.
You'll need a 3.5mm audio cord with two rings. My phone's old headset wouldn't work because of the built in mic, so it has to look like this.
I went to China-Mart to pick up the cheapest, thickest adapter cable I could find. This thickness helped with splicing. It has typical insulation for cheapness, and the coloring's simpler. The black one was two bucks, clearance aisle. **Alternatively, you could use anything with a two ring 3.5mm plug like old headphones or speakers. Just understand that you are salvaging the cable only and there is no guarantee the wires inside will make it easy.
I only did this next step to verify the color and location of the wires on the XM receiver harness.
I pulled the XM receiver from the the rear drivers side wheel well, in the trunk. I had to unbolt it to remove both plugs. The blue plug on the left is power. The right plug is the signal wires, and you view the pinout. There's 7 total wires. The taped lead is the "Drain" wire on the trunk side. This will not be present on the dash-side of the harness. (Sorry for the fuzziness)
Now back into the car. After you have the radio out, there are two head unit connectors on the harness and the antenna lead. (Note your harness won't have the bright green colored wire between the plugs. That's power for my boost gauge LED.)
This is the plug you want. Hmm..only 6 wires on this side. (remember that drain wire?)
I cut off some tape and separated the wires the way they made sense to me. Testing showed I chose a ground correctly, but I think the black wire would also work. There's two groups of three wires. The left side of the harness is the wires we want. They're tan (the lighting is off), dark green/white, and brown/white.
I used an xacto and removed a bit of the insulation.
I decided to run the wire through the same factory hole reamed a bit for the boost gauge. It's in the storage underneath climate contol (EDIT: This caused a ground loop noise when the rear defrost was on. I moved the cable away from the HVAC controls and added a ground wire to the head unit.) The new AUX cable isn't pictured, but I ran it through here before splicing.
All spliced in after making sure I had the left and right channels correct. The tan wire is a common ground.
(I went back and soldered each splice. Lol)
Ready to use! I only needed about two feet of 3.5mm adapter cable.
Again, this only works if your car has an XM receiver. I spliced into the harness and tried without the XM plugged in the trunk. The head unit wouldn't select the XM band at all.
You'll need to adjust your headphone volume and tune it to XM Channel 0. I'm sure other signal sources, like a rechargeable bluetooth receiver will work just fine. You could wire in a female 3.5mm adapter instead to avoid cutting the ends off other things.
Thanks!
