AMP INSTALL on 2007 Ford Mustang

I have a 2007 Ford Mustang, i am trying to route the power cable to the engine compartment to hook to the battery and am not sure where the hole is that has a rubber grommet, i read its somewhere in the firewall but not sure where exactly. can anybody help me?
 
passenger side firewall, kind of up behind the dash. There is already a big wire bundle going through the grommet. You will have to take the front tire rear splashshield off to get at it from that side