Instructions
Play through the game until you reach the boss fight with Grunthilda at the end of the last "Terrarium of Terror" level. Use the laser to defeat her relatively easily. You'll receive a horn upgrade for your vehicle after you defeat her.
Go to Mumbo's Motors to modify your vehicle. Attach the horn to it and then go back to the central game hub. Press "Left," "Right" or "Down" on the directional pad to assign the horn to either "X," "B" or "A," respectively.
Go to a crate that has a ghost pig sitting on top of it. Press the button that you assigned to the horn to activate it. The horn blast will scare away the ghost, leaving the crate free for you to bring it to Mumbo's Motors.