Using a DynamicMaterialInstance created in the construction script of the vehicle to change value does not apply the change to every other vehicle. With that created you then use a “Set Scalar Parameter” node from the event graph to turn set the emmissive strength to turn it on or off. will only affect that instance of the vehicle, using a regular material instance will affect them all, but not a DynamicMaterialInstance.
You can see in action by placing multiple copies of the truck in the VehicleGame sample, each one is independent.