RORY: We – we went to dinner and then we walked by the bonfire but it wasn’t lit so we went to this junkyard and we sat in this car and then – oh God!
RORY: I forgot your meatball in the car.
This is the meatball that Lorelai said to save as a memento. It is typical of Rory to get hung up on this very minor detail of things going wrong. Notice that Rory says they just sat in “this car”, rather than letting Lorelai know it was a car that Dean was building for her.
Rory didn’t actually have the meatball in the car Dean is building for her, but possibly she means the car Dean drove her home in.