Well, here in Minnesota anyway, it’s not entirely uncommon for someone’s jackass friends to think it hilarious to hide a dead fish in someone’s truck. Grumpy Old Men got it wrong though - you don’t just toss it on top of stuff, you conceal it. One guy I heard of tied it to the back of the brake pedal so that it would squish and get extra smelly every time the driver stopped. My friends are aware that their lives are forfeit if they ever do this, but perhaps the previous owner of your truck wasn’t able to be so convincing with his friends 
I’d start by trying to locate where the smell is coming from and work from there.