I had a similar situation in the past. If you can't get an A/C in there, you'll have to keep the garage door half open with a fan circulating the air. Have the fan blowing out, and a fan blowing on the rats. Also, if the attic door is located in the garage, keep it open to let the hot air out in that direction too. It worked for me in regards to keeping the rats alive, but their reproduction rate was severley hindered. Good luck.
-Marc