I found the clip. It's number 6, "Convoy of Death" starting at arond 8:35.
It turns out it wasn't a car he was sitting on, it was a lawnmower. Having viewed the clip, I'm led to believe that he "sat on the bonnet", the bonnet being the front of the lawnmower. I can't say I know much about those sit in, drivable lawnmowers (Outside of spefic businessness and the landscaping profession, they're not very common here), so I can't say for sure if I was right or not about it being the engine cover or if it's actually an American term used to label the front of such a lawnmower spesifically. May there is a connection as the front of a car is usually whare the engine is found, but I don't know.
Either way, the "bonnent" in this context is the front of a lawnmower. So... yeah...