I gave that a try on my 07. Got a loud pop everytime the radio was turned on. Disconnected the sub. You can get a resistor/regulator for the line to drop the voltage to 5 volts.
12-5 transistors are about $1.00 at Radio Shack. Solder the amp power-on (in) wire on (the wire that the harness would attach from the head unit to the amp, which usually is blue/white), solder a ground, and solder the power-on (out) wire on (the new wire to the amp). Heatshrink the connections for added protection. I mounted my transistor to the right of the radio (inside the dash) on one of the metal dash support plates. It already has a hole in it, and using a small bolt, lock washer and nut, I secured it. (You'll need to open the glove box, fold it down, and reach up and into the dash to hold the nut as you tighten from the radio area.) Ground the middle wire anywhere; I did mine to the metal underneath the center console.
EDIT: The link below was very, very helpful in my install. I did exactly as he did, including the four-step heatshrink application.
