For most vehicles, only one extra can be added so both the second and third parameters should be set to the same wanted variant to avoid confusion. However, some vehicles can have 2 different extras active.
Variant -1 is the default model with no extras. Variants 0 through 5 are possible extra variants which will fall back to the default if the model does not have that many extras, and variants greater than 5 will crash the game.
1: Munky Juice
4: RS Haul