I am assuming that the fire wall setup in the 13 ST is the same as all mk3 focii.
Your ST will not have a convenient place on the drivers side firewall to punch through because the place that some others on this forum have used is taken up by your clutch assembly.
I also have a manual transmission on my car, so I went through the main power feed grommet on the passenger side. Open your hood and look towards the rear of the engine bay. About 6 inches below the top of the compartment and about 6 inches to the right of the passenger side wall will be a rubber grommet with a cable wrap going through.
You will want to carefully make an "x" with a knife or razor on the side of the rubber grommet and push your amp power cable through. Do not take the easy way out and pull back the rubber grommet and slide the cable between it and the frame, as this will lead to the cable rubbing against the car frame and eventually wearing through the cable insulation and causing a short and possible fire.
Let me know if you have any other questions.