2003 Silverado, amp to factory head unit

Hi I am trying to hook up an amp to the factory head unit and need help locating a constant 12v line for the turn on lead. Does anyone know where I can locate a 12v constant to get my amp to turn on when the car starts? I heard at the back of the factory radio one of the wires is the turn on lead for the radio so I can just tap into that to use for my amp but some people have also said there isnt one. let me know if there is or not and if not where else can i tap to power my amp?

The fuse box is the easiest if all you need is just to switch on your amp. Get yourself a fuse tap so everthing will be neat and tidy. Be careful whose advice you buy, but be patient with those who supply it.