2001 jeep grand cherokee amp

I am installing an Amp and two subs in my Jeep Grand Cherokee and wanted some help. It's a 2001 and has the infinity system with a cd changer. When installing the amp, which color wire do I tap into at the HU? By this, I mean the wire that tells the amp to turn it when the ignition is turned(maybe called ignition wire?? I can't remember). Also, I heard from someone that I actually should tap into this wire at the stock amp that's already installed instead of the head unit. Is this correct?

actually, I meant the remote-turn on wire, thats the wire that I wanted to know the color of.

Ignition on this car is RED / white behind the radio (meter it to verify it 100%), but ideally, yes finding the facotry amp trun on lead would be the best.  

ok, i'll give it a try and let u know what happens, thanks

I am having trouble finding a hole in the firewall that I can run my power wire through. Any ideas where I can find such a hole?