If the 2006 Highlander has a power steering fluid leak, what is the cause?

From time to time in the 2006 Toyota Highlander (and in many other vehicles), You'll notice issues with the power steering system. Some of the most common problems include low PS fluid, air in the PS fluid, or a failing power steering pump. For the sake of the "leak" modifier, we'll ignore a failing power steering pump for now. If You have low fluid (or if You see air bubbles in Your fluid), it can cause Your power steering system to run loudly and inefficiently. This can create more problems down the line, so it's suggested You find the source of the leak as soon as possible. The two main offenders The gasket around Your fill cap, and a split or crack in one of the lines themselves.