First myth to dispel is the ram air effect of a trumpet, you won't get anything useable at the speed that car can do, the engine will likely be breathing faster than any rammed air, so long as it is open to cold air it's fine to just route behind the bumper, I wouldn't worry about the amount of hot air it would be able to pick up from being close to a rad hose, you seriously would not feel a difference.
tabetha