I don't know if this will help or solve anyone's problems but here it goes.
For the last month the volume down button on the steering wheel in my 07 Silverado (non classic) has been working intermittently. The volume up button would sometimes stick, the mute button rarely ever work, and sometimes the source would change even though I didn't hit the source button.
First thing I did was take it to the dealership and had them replace the steering wheel controls.
That didn't fix it.
Then I emailed PAC for help. During our emails I discovered a service bulletin for the wiring harness adapter that I purchased in order to hook up my Z2, I have the OS-3. The problem is with accessories (ie head unit) not turning off when the key is removed and door opened. Or the chime not turning off when the key has been removed from ignition. I had both problems every so often. The bulletin suggested soldering jumper wires across the connector between the OS-3 and the vehicle's harness for the GM-lan, positive, and ground.
Now, what does this have to do with the SWI-PS? Well I used the +12v and ground coming from the OS-3 to power my SWI-PS. I also suggest checking the relay that came with the adapter to see if it has a diode across poles 85 and 86. If not you can potentially back feed power through the +12v accessory wire.
I made the repairs to my vehicle yesterday and so far I have noticed that my steering wheel controls are much more responsive. Not only that, they all work now and the stereo turns off when I open the door. No more open close, open close.