You don't need to drill the firewall if I recall correctly there's an extra hole in the rubber grommet where the throttle cable passes through the firewall, that would accomodate your main feed to the amp. And routing that under the carpet won't be too much of a chore, just pull up the plastic trim that covers the edges of the carpet & the rocker seams on the driver's side (in the door sills) and you'll be able to pull the edes of the carpet back enough to route the cable alongside the main loom for the taillights/fuel pumps/fuel gauge sender.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf