What object should i use for this cast?
You need a reference of your Car. But you are asking a pretty simple question about Casting, so I recommend watching this fantastic video about Blueprint Communication! <3
If there is ONLY one car in this game, and you want to make it quick and easy. use GetAllActorsOfClass (get BP Class), then Get 0 from that array, and use that as the reference.
https://docs.unrealengine.com/latest/INT/Gameplay/HowTo/FindingActors/Blueprints/
I would definitely consider this a “cheat” way to do it. But until you learn more about how to properly get references, this isn’t a bad way of doing it really. Definitely say you should watch the video @Ninjin linked though.
Are you still having troubles after watching the video? Would be great to hear if you solved it, otherwise don’t forget to