It seems that the wiring harness, and the whole 3rd brake light assembly are no longer available. Even my local Chevy dealer says they can no longer get them.

So, there are a couple of options for repair / replacement of 3rd brake light.
1> Junk Yard / U-Pull-It yard - difficulty here is that the battery is probably dead so you'll have to crawl into the back & hope you can pop open the rear lift gate, or bring a portable jump battery with you to temporarily supply power. Also, what you find might be in as bad or worse shape than what you're trying to replace / fix...
2> ebay but you have to be careful.... watch out for cut wires, seen a couple where the car side connector is still attached to the assembly in the listing photos.... could be nothing, or it could be rusted to the assembly connector.
3> DIY - Do It Yourself

I LOVE DIY options, almost always cheaper, and as long as your careful and thoughtful, often just as good as original, or sometimes even better.... Having just got another 2010 HHR, this one an LS to replace my LT due to broken transmission housing (that's in another thread), and finding that there's a bit more work to be done with it... one of which is the 3rd (or high) brake light.

After attempting to replace just the bulbs, it became apparent that that was not going to work. 2 bulbs came out, but the sockets are extremely corroded, while the other two bulbs will not come out without breaking the bulbs. Days of searching and dozens of phone calls later, I came to the inescapable conclusion that I'm not going to find replacement parts locally. None of the junk / U-Pull-It yards within 15 miles have an HHR to scavenge, nothing at any auto parts stores, and dealers cannot get them. ebay has lots of assemblies listed, but ebay is a last resort for me & at an average price of $58, definitely more than I would like to spend.

SO, ordered some parts, once those came in, I pulled out my soldering station, grabbed two spools of silicone insulated 18AWG wire & got to work. First made a mock-up of what I would do, using Wago lever lock connectors for speed & simplicity.



For replacement lamp holders, I got these from Amazon: While they are not an exact fit, they do fit and turn into the light assembly. ONLY use them for LED bulbs, they should NEVER be used for standard halogen bulbs, they are soft plastic and they will melt from the heat of a halogen bulb. I've yet to find a heat resistant holder that actually fits the HHR 3rd brake assembly, that doesn't cost $30 per holder...

Wiring is pretty straight forward. Pull the old wiring out the the assembly, insert the new holders as they will be into the lamp assembly, and decide whether you want red or black to go to the striped wire. In my case I lined them up so that the black wire goes to the striped wire.

Originally, I was going to just solder these together, so that I could later de-solder and add resistors if needed to make the cruise control work with LED bulbs. However, there are LED's out that will still work with the cruise control according to a few posts I've found. Osram has one for sure, and I'm sure there are others.

Anyway, once I had everything lined up, I decided to use solder butt connectors. These have a very low temperature solder ring inside of a heat shrink tube. I used the hot air portion of my soldering station and a 1/8" diameter nozzle to direct the hot air exactly where I wanted it. First shrinking the tubing around the insulated portion of the wire on either end to ensure that it's held in place where I want it, then work on the solder ring rotating around till I get a nice even flow of solder inside the heat shrink for an absolutely solid connection.

Here's the new almost completed harness next to the lamp assembly:


All that's left from that point is to cut off the old harness connector flush against the first bulb holder from the GM harness (leaving yourself as much wire as possible to work with), strip the wires & repeat.


I inadvertently skipped the planned "test step" where I was going to use those Wago lever lock connectors to make sure it all worked before making the final solder connections to the connector end. But having everything planned out ahead of time, that wasn't an issue, and the new harness works as expected! Here I am testing it out in the LT.

Works as expected so far - time will tell if these bulbs work for cruise control or not...

NOW, I just need to find where the break in the wire is in the LS. It's not a fuse, all fuses are perfect, and ALL of the other lights still work, tag lights, backup lights, marker lights, interior lights, L/R brake & turn lights. It's only the 3rd brake light that's not working in the LS.

Overall cost about $15 if I use the SirIusLED bulbs that were $1.20 each, about $25 using the Auxito bulbs in the test photo, or about $45 using the Osram bulbs that are 'known' to work with the HHR cruise control.

 

Hint: to open hatch with dead battery take a memory saver that plugs into the cig lighter. Or, a 9 volt battery and 2 jumper wires to connect to the under hood fuse box.