https://www.civinfo.com/threads/how-to-install-a-subwoofer-and-amp.399/

"Next thing was the power cable from the battery, and let me tell u it was not easy trying to find a way into the car from the engine bay. but there is one, right in the middle at the back of the engine, all the electrical wires run through a rubber seal into the car, It was difficult to get a picture of this and this is the best one (sry) but you can just make out a bunch of wires and my red power cable.."

 

I just wrote a "How to" just on how to run a cable through the firewall. It'll be approved in couple of hours I hope.

I don't like the option above.