More likely a mechanical problem involving frayed/broken/loose wires. Or, somebody mixed some wires up.

 

Don, unlikely on the mixed up wires per say. The ONLY thing ever aftermarket done to the car was adding a trailer hitch and wiring 5 years ago BUT that does deserve some investigation since they're 'tapped' into that area. Will check fuse 69 tomorrow and if that doesn't work, will temporarily remove hitch wiring. I did the hookup 5 years ago but it certainly could be the culprit. No relay installed; used LED lights. The more my old brain ponders this, the more I'm thinking that may end up being the culprit.

 

LED lights are transistor switches that happen to glow when energized by current. If they are not wired correctly, they can cause havoc. If the wires were tapped into wires under the dash, they could be loose and touching thing they shouldn't. Especially if the person doing the wiring doesn't know that the brake pedal sensor is not a on/off switch like they used to be last century.
Trailer light control modules are really cheap and easy to install.
Like this one:

 

RJ and Don Brew and everyone else,

PROBLEM FIXED!! Turns out fuse 69 was the culprit (short version). Didn't get a chance to do any more diagnosing till just an hour ago. Started with the hitch wiring, which I temporarily disconnected. This was a Curt hitch I installed 10 years or so ago which has a very nice 'splitter' for the HHR. You disconnect the connector from the left taillamp and connect the adapter which gives you the breakout wires for the trailer connector. That didn't fix the problem so I moved on to the fuse 69 idea. DUH! pulled the fuse and could immediately tell it was blown. Tested it anyway and no continuity. Replaced 10 amp fuse with another and YAY the taillamps worked when the headlights were turned on. Won't know till dark whether the other problem involving the brakes and radio cluster illumination is fixed but pretty sure it will be. Will follow up with another post as soon as I get a chance to try it out in the dark. AGAIN, thanks everyone who offered suggestions. Didn't think it would turn out to be so easy of a fix.

 

 

Huh. If you didn’t return the old one, you have a perfectly good spare battery. You could go buy another HHR with a bad battery to put it in.

 

So, it was the trailer harness; the parking lights fuse confused the black box.

 

Confirmed last night that everything was back to normal. Really won't know if it was anything in the trailer harness wiring until I reconnect it but only expect to use the trailer once or twice in the next 6 months. Happy to have replaced battery since I plan to sell the HHR within the next year. Have more cars than we use and high on the list for a 23 Ford Lightning if I can get a work truck this time around. No interest in spending 70k or more on a truck.

 

When you need a trailer harness again, you might consider starting fresh and not simply reinstalling the old one, certainly not in the exact same way it was before.

The good news is modern LED trailer lights can reduce disruption to the tow vehicles electrical system because they draw so much less power.

 

I installed this from Curt


Took about ten minutes, no cutting the OEM harness