Use a small knife to poke a hole right next to the opening where all of the other wires in your vehicle feed into the vehicle. This is the wire that your head unit uses.

Step By Step Instructions For Wiring An Amplifier In Your Car
Where to run remote wire for amp. Youll have to remove the stereo to get to this wire. On aftermarket stereos its usually a blue and white wire. To connect wire to wire an appropriately sized butt connector or other style connector can be used. On most vehicles the wires run to the glove box on the opposite side of the engine. The remote wire will tell your amplifier to turn on whenever the stereo is powered up usually whenever the vehicle is turned on. Poke a hole in the rubber next to your firewall to feed the power wire through.
When you shut the car off the wire loses power and the head unit and amplifier will both turn off. Simply connect another piece of blue primary wire to the blue wire in the wiring harness and run the primary wire back to the amplifier to terminate in the remote turn on terminal. The cheap and easy way is to run the amplifiers remote terminal off of a switched accessory wire. It is only powered when the key is in the accessory or run position of the ignition switch. The turn on wire also called the remote wire is located behind the stereo.