Objects on an Isometric map

Hi, Well, should the tiles on an isometric map, know what objects are on it, or should they not, or should the object know what tile it is on? Or should both know? The reason I ask is, that I was wondering how you should calculate what order to draw objects in. Not the tiles, but objects, such as maybe a Car, like in Simcity 3000, or a building. How do you figure out the order in which to draw the objects in as little time as possilble. P.S. I realize that you should draw them from left to right, top to bottom. Thanks