I do not allow SS8 and 9 to eat in my car at all anymore. My SO even set the same rule for his vehicle (unless the situation requires it). I got so disgusted when I kept finding old food in my car. The worst was probably a melted half eaten poptart and a chicken nugget that were a month+ old.

Car, mobile restaurant or trash can for the skids to abuse?

They have no respect for anything! Do not let them eat in your car. Eeeww!
Worst I ever found was a melted cheese smeared on the back of the passenger seat. By the smell!!
Of course it wasn't older SD.
 

SD brought toast with her from Waffle House about a month ago wrapped in a flimsy napkin. She finished eating it on the way to church. My leather seats were freshly vacuumed. I look back after she got out and crumbs all over the f-ing place. This kid is 12! No food EVER AGAIN unless it is sealed.

DH's car is like that - add empty pop cans.

The few times the skids were allowed on my car, beverages were the only thing allowed and the container got out of the car with them or, the next time, they hgv and to finish/pitch it before they got in.

Yeah. I'm a batch like that. My car; my rules. 

No food in my car. Too many spills and no one cleaned up. DH let SD14 take food on a trip once and there's a grease stain on my back seat now.  I'm really vigilant about it. Water is ok .

Sometimes I found miscellaneous stuff left in my car, trash from opening stuff like after they got new clothes or socks or pens. SD12 always cleaned up but not SD14. So I used to throw SD14s trash she left into her room. She can deal with it. One time i just left a pile of stuff (after DH borrowed my car) out in the driveway.