It's not a difficult job to sort that tbh. Just need to remove the fuse or trip the circuit breaker, disconnect the power cable from the amp, pull it through from the engine bay, feed it through the grommet in the bulkhead into the cabin, fit it under the trim/carpet and then connect it back to the amp.
I almost hate to ask, but there is a fuse or a circuit breaker near the battery terminal (as in a couple of inches tops) isn't there? Please tell me there is... [-o<
Just take this to the place and ask them to follow those instructions, shouldn't take long. Best of luck.