OK, the units they sell are specially made to fit their harness and there could never be 2 manufacturers using the same one.
(I don't think so)

There is no switched power in the car harness that is why you need the adapter along with the chimes. The chimes are generated by the BCM and played by the radio.
Any of the spare fuse locations on the BCM can be used for switched 12V and they are on the retained power circuit.

I have never heard of a substituted radio disabling a HHR. It should only disable the radio.

BTW, crutchfield sells Dual radios. Here are the install kits including the antenna adapter. https://www.crutchfield.com/fg_300_F...?tp=5684&avf=N

You only splice on the radio side.

More search options available if you pay me to do it for you.

 

When I opened that link Donbrew posted, I keyed in make model and year, no Pioneer, no OnStar.
Some Dual radios are compatible and some aren’t, which model radio did you purchase at Walmart? It might be one of those that isn’t compatible.

https://www.crutchfield.com/fg_300_F...r-Stereos.html

 

I have no idea what would make a radio incompatible, maybe a weird DIN standard or too big for the hole. The Harness is a GM to generic harness with the GMlan-03 attached, that is available at many retailers along with the antenna adapter. https://www.oreillyauto.com/shop/b/a...6/c74b90a2b820
All 6 of the Dual HUs I see at Crutchfield say "fits".

Waste your time and money if you wish but, a little time on GoOgle will make your life easier. Listening to sales people on the telephone when you are clueless is a waste of time. They are looking at the same website you can look at.

https://www.chevyhhr.net/forums/audi...et-radio-6158/

 

I put a Sony in our HHR from Crutchfields Used a small solder iron and shrink wrap for all the wires. Pretty simple. I now have Pandora. USB, CD and radio. It supports sirius XM and others Blue tooth too. On sale it was under $100. Highly recommend Crutchfields. Their support staff will help you with anything you cant figure out.