Spawning vehicle parts

To spawn custom vehicle parts you first need to get the item.

The spoiler items in my example pack are leather leggins, below are some spawn commands:

/minecraft:give legofreak107 minecraft:leather_leggings 1 1 {Unbreakable:1}  
/minecraft:give legofreak107 minecraft:leather_leggings 1 2 {Unbreakable:1}  
/minecraft:give legofreak107 minecraft:leather_leggings 1 3 {Unbreakable:1}  
/minecraft:give legofreak107 minecraft:leather_leggings 1 4 {Unbreakable:1}  
/minecraft:give legofreak107 minecraft:leather_leggings 1 5 {Unbreakable:1}

When you have these spoilers. You can use this command to make it into an addon".

/addon givepart

Now you have the item. Just click the vehicle with the item.