where is this relay located on 2007 hhr?

be more specific. Relay for what, and what are you trying achieve?

Especially since the only relay mentioned had to do with one of the posters 'old Colorado' and wasn't part of the OP's solution.

I just bought 2007 HHR LS and having the same problems with temperature gauge and air conditioner. I already replaced the coolant sensor, didn't work, so today I am having the thermostat replaced. Is this the same as mentioned in the above posts?

T stat should fix the issue, ask the mech to use the correct GM stat! The ECM isn't happy with any of the other aftermarket ones!

Heat gauge and AC not working all of a sudden
 

Same thing happened to my wife, I found the relay in slot 15 in the fuse box under the hood was the problem, switched it with 18 to test and as soon as I plugged it in I heard the
AC motor kick on, quick trip to the auto store when its open and it will be good as new.

The same thing happened to me this weekend. On Friday morning, the A/C quit working all of a sudden. Half way home on Friday afternoon, it came back on as magical as it quit. It worked fine all weekend, but just a little while ago, it quit again, but this time I noticed that my temp gauge is not working at all and my digital readout is dashes. I was thinking that it may be the temp sender, but someone I read changed it without being the problem, I will try the relay as JTP suggested. thank you.If for some reason the relay does not do the trick, does anyone know where the temp sensor is located? I would appreciate it. Thanks.

If the temp gauge has flatlined, then most likely it's just a failed thermostat, I posted information in your other thread

Thank you Old Blue, now I need to find my other thread. (I am new to this) :)