a) for a police officer/detective/Sherlock Holmes or anyone along those lines to be in hot pursuit of either a criminal (usually a juvenile delinquent truating)
b) for a civilian in search of new unplanned adventure in the great outdoor wilderness.
c) can also be used as a derogatory term for a couple who have sex outside in public (usually on the bonnet or hood of their own parked car).
d) to be in the middle of doing something important
officer a): where's my partner Michael?, you seen him?
officer b): i think I saw him on it like a car bonnet, chasing after that hot female teenage delinquent.. (moments later): Tom, here goes the answer to your question, there's Michael, on it like a car bonnet; i don't think we should be disturb him (pointing at a car parked some distance away). He is on it like a bonnet, raping that girl, instead of giving her a hefty fine. I hope they become a cute little couple, lol.
officer a) Thanks man, well looks like that girl didn't have the money to pay the fine anyways, so she got what she deserved. Let's just hope she doesn't have a beautiful disaster. Hang on partner, let me call me my wifey, tell her aqbout Michael and his big fish.
officer b): don't call your wife with such nonsense, you buckwheat brains; she may be on it like a car bonnet, probably playing bingo with her girlfriends or whatever.
Some say it is derived from Essex girls famed love of sex on a car bonnet, others from the female models at car trade shows that lay on car bonnets. (NB: UK Car Bonnet = US Car Hood)
To be "On it like a car bonnet" means you are literally all over something, like an (Essex) girl on a car bonnet"
"Yeah, I'm on it like a car bonnet"