Wild Ride

“Harmonize [cost]” means “You may cast this card from your graveyard by paying [cost] and tapping up to one untapped creature you control rather than paying the spell’s mana cost,” “If you cast this spell using its harmonize ability, its total cost is reduced by an amount of generic mana equal to the tapped creature’s power,” and “If the harmonize cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack.”

You can cast a spell using harmonize even if it was somehow put into your graveyard without having been cast.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a harmonize cost) you’re paying, add any cost increases, then apply any cost reductions (such as the cost reduction from tapping a creature). The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.