To use an amplifier with your 2008 Ford Focus, you'll need a line output converter (LOC). This device converts the low-level signals from your head unit's speaker outputs into higher-level signals suitable for the amplifier's input. You'll also need speaker wire to connect the LOC to your head unit and RCA cables to connect the LOC to your amplifier.
In short, you'll need:
* Line Output Converter (LOC): This is essential.
* RCA cables: To connect the LOC to the amplifier.
* Speaker wire: To connect the LOC to the speaker outputs on your head unit.
* Amplifier: The amplifier itself.
* Speaker wire: To connect the amplifier to your speakers.
Remember to consult a car audio professional if you're unsure about the wiring process. Incorrect wiring can damage your car's electrical system or your equipment.