Assuming you're using Sprite Kit, you can use the SKNode method convertPoint:toNode:
to convert points between coordinate systems.
So for instance, as soon as dragging ends on top of the vehicleNode:
CGPoint iconPos = iconNode.position; // this is position on shelf node
iconPos = [shelfNode convertPoint:iconPos toNode:vehicleNode];
// ... remove iconNode from shelfNode
// ... make new decorationNode
decorationNode.position = iconPos; // this is now position on vehicle node
// ... add decorationNode to vehicleNode