Create a sphere or a box component in your vehicle BP. Make it large enough for your character to overlap since you’ll use it to generate overlap events so that your character can posses the car only when you are near it.
Then go to your player controller BP, Event begin play > Get all Actors of Class(your vehicle BP) > ForEach Loop > Cast To(your vehicle BP) > Set it as a variable. Do the same for your human character and then setup your spawn and posses events using those variables and the overlap event of the sphere(or box) you created in the vehicle BP.