06's do not have dome lights that can be turned on with a switch, unlike every other car. You must open the door to turn on the interior lights. It does how ever have map lights in the rear view that can be turned on.
06's are notorious for squeaky front strut mounts and front sway bar end-links, in cold weather. Mine have been replaced twice. Some VERY early 06's, 05 build dates, have leaks around a gasket in the firewall on the passenger side of the car. You can have a pound on the passenger side floor. Most were recalled, and fixed.
The HHR shares a LOT of parts with the cobalt. My HHR is lowered on Cobalt SS springs.
I would also check the rear quarters for sandblasting...from not having running boards. Mad flaps help, but do not fix the problem. Some 06's have a plastic peice installed to counter this debris that is flung up by the front tires onto the rear fenders. GM will repaint the car, once, and advise you to purchase running boards.
So if the HHR your looking at has that. Ask the dealer about it. There is a TSB on it.
I love my HHR, and it has treated me well. It makes a great daily driver. I get around 30mpg, but mine is a 5 speed. The stock tires are OK, at best, in the snow. If you live in a snowy climate you might want to purchase some snow tires or new all sessions.
If you have any questions feel free to PM me.