I found the story that the sentence comes from. If you are reading this story, then I think you can find the answer from the context of the story. But to help you, I will say that in many contexts the word "odor" refers to something that smells bad. For example: body odor. Unless there is something in the context that tells me that the odor refers to something that smells good, I would assume that the odor refers to something that smells bad. (Also, the author mentions that the boy looked at the handle to the car window; but the handle was broken, This means the buy could not open the window for fresh air from outside.) The author uses "sweet" and "stale." It just means that the inside of the car smelled somewhat "sweet" and somewhat "stale." The words "sweet" and "stale" would have their normal meanings here. But remember that not all things that smell sweet are pleasant things. So, the author may be referring to the smell of the car in general. Or the author may have some specific things in mind when she mentions that the car contained a "sweet stale odor." When I read this passage, I find that the author lists *two* specific things that probably make or contribute to this "sweet stale odor." Can you find these two things in the story?

It does mean a sweet odor; exactly as written. A sugary smell, but dry and old.

More context is needed to give you a more certain answer. But "sweet" usually means something pleasant. But sometimes unpleasant things can have sweet smells. For instance, I have just read in a novel that some people find that the smell of blood is "sweet." But this does not mean that the person thinks that the blood itself is something pleasant.

I cant imagine this but i think its mean old sweet smell

Sweet = sweet. Just sample my wife's car. She leaves food, biscuit & candy way too long in there. It stays closed for the weekend, then open the door, on a Monday morning, a sweet stale odour.