Please make sure that you never, ever keep your guinea pig in a closed garage with cars that have just been running. The heat from the engine can make it several degrees hotter, not to mention the possibility of carbon monoxide poisoning with no ventilation for your tiny pigger's lungs.
That little fella needs out of the garage **IMMEDIATELY.** He's being served a death sentence, and it's not fair.
Is there any way you can:
- temporarily house him in a box/smaller cage from petstore INDOOORS where it is cool and non-toxic?
- get him out of the garage and indoors now?
- buy alternative housing materials and build a cage? C&C is always best, but there are endless other solutions at your local hardware store.
Please keep us posted. I've been very concerned about this guinea pig. I'm glad that you are at least trying to do the right thing.