To connect an amplifier, you need a high-to-low level converter or a line output converter (LOC). This device takes the low-level speaker-level signal from the factory head unit and converts it to a high-level RCA signal that the amplifier can accept. These are relatively inexpensive and readily available.
You'll also need:
* RCA cables: To connect the LOC to the amplifier.
* Speaker wire: To connect the amplifier to your speakers.
* Power wire kit: To power the amplifier (this involves running a power wire from the battery to the amplifier, with an appropriate fuse).
* Ground wire: To ground the amplifier.
The installation requires some basic car audio knowledge and tools. If you're not comfortable with car audio installation, it's best to have a professional install the amplifier and LOC for you. Improper installation can damage your vehicle's electrical system or lead to poor sound quality.