OO code and movie clips on the timeline

My group builds apps by creating movie clips which are precisely positioned on the timeline and adding an action to the first frame of the movies’ timelines to include external Actionscript files to define properties and functions. This works fine for building self contained objects, but doesn’t seem capable of taking advantage of object oriented design principles, like inheritance. Ideally, I would like to build objects, like cars and bicycles for instance, and have both inherit code from a parent vehicle class. Any suggestions on how I can do this given the movies are already instantiated on the timeline?