You need to somehow get a reference to the Car_UI widget. Not sure where this widget is located (3D widget component, viewport etc) but that will determine how you get the reference. Post some more screen shots of how you create this widget and where it is located and I will be able to give you a better answer of how to reference it. You can also check out video #25 in this series about creating references that might give you some tips.
This video also shows how to reference a 3D widget component (albeit from another widget) but you can use the same method to reference a 3D widget in any BP if your widgets are 3D components of other actors.
