Fitting Sub/Amp into A4

hi all, i intend to swap over my audio from my old car into the audi

Ripspeed DVD head unit (yes i know Ripspeed! but it was a cheap monitor unit)

Pyle 2-channel amp (will be getting changed eventually)

JBL 10" sub

i also plan to ungrade all door speakers

first question, before i start ripping the dash apart to find the bulk head, where is the best place to feed through the 0awg cabling from the battery into the cab?

also, i noticed that there is already a small sub on the parcel shelf, disconnected, can this be wired up also?

many thanks