Where is the best place to get through the firewall to run the power cable?

Where is the best place to pick up the 12V, ignition-activated, amp turn-on signal? I'd prefer a place somewhere in the area of the stock head unit, since I plan to place a switch in that area to control the amp from the cockpit. A specific location and wire color would be greatly appreciated!

..and keep in mind it's a 2001.

Thanks!