If you win a car you already own from a Wheelspin, sell it in the Auction House rather than deleting it.
Once you have successfully achieved 3 stars at the Lakeside Speed Zone with a Swedish vehicle, the game will reveal a rough location for the treasure chest on your map. forzahorizon4ultimateeditionlootboxpart08 link
Purchasing the legitimate Ultimate Edition grants you the complete, definitive Horizon festival experience without risking your PC's security. It includes: If you win a car you already own