We hada load test done is how we found out it was the stereo. It is is odd I have installed numerous after market stereos in gm products. Only 2 have I had the issue of the yellow acc wiring missing. This HHR and an 08' Silverado. I am sure I wired the HHR as I did the Silverado and the Silverado has had no issues of battery drainage. Since they said it was the stereo we disconnected it and sure enough the HHR has had no other issues electrically. We are planning on just having a professional reinstall it. We know the original stereo was shot, and the speaker had been blown and wires for the speakers removed so I had to rerun those too. It was a mess to work on for sure.

 

You did not load test the battery and find a problem with the radio.

 

The HHR has a GMLAN controlled radio. There is no "ACC" connection to the standard radio. An after market radio requires a "black box" in order to connect to the GMLAN and have warning chimes. An alternative is to wire the ACC wire to a switched fuse on the BCM, any of the NC fuses there will work, in that case you will lose warning chimes and steering wheel controls.
I sense that you are under the impression that all GM vehicles are identical. Wrong.
I don't know how a battery load test would point to any drain source.
Unless you are using OnStar or XM/Sirius pull fuse #24 on the BCM (fuse box under radio passenger side). That might be drawing power.
All radios I have ever seen have constant power AND switched power of some type. The HHR has switched power via the GMLAN computer network. Some complete boobs wire the after market radio directly to 12v so you have to turn it off and on manually.

 

An awful lot of HHR radios have replaced because the owners didn’t realize it was the crappy base speakers that had all failed.

 

That is so true. When I lost 99% of the sound I first thought it was the radio. But like you said it was the speakers !