Look inside your trunk. There should be a panel or trim piece covering the area where the wiring and light fixture are located. This panel is usually held in place with clips, and may require some gentle pressure or a trim removal tool to release it without breaking the clips. Once the panel is removed, you'll have access to the light bulb.
Important Note: Before you start, check your owner's manual. It will provide the most accurate and detailed instructions specific to your vehicle. This is crucial to avoid damaging your car. If you don't have the manual, you can often find a PDF version online by searching for "[Year] Ford Focus Wagon Owner's Manual".