Although its a lot more work, I do believe you can run wire from the battery through the fender area into the cabin area. Its basically where the hood latch cable runs.
I thought about doing it the way I mentioned above, but you would have to either take the fender off or blindly navigate through.
So what I did was: I cut a slit through the rubber that, if I remember correctly, the steering column runs though. Or maybe there were some wires running through it. I don't remember, I haven't been down there in a while. Its a large circle with a hole in the center of it, the center hole sticks out into the engine bay.
I cut a slit from the engine bay side with a utility knife, and I ran a 4g wire and a 0 g wire through. From there the 4g wire goes toward the passengers side to my front speaker's amp. The 0g wire goes over toward the fuse box, then I managed to fit it into the fuse box and it goes down under the carpet, and its wedged into the slit below the door, next to where the pull is for the hood release cable/trunk release cable. Theres a small tunnel thing you'll have to go through (where the seatbelt is i believe). Then it goes from there to the backseat area, under the seat itself (which is easy to remove), into the trunk where my sub's amp is.
And on a side note, the remote wire is a POWER wire, it does not have a noise signal in it. Therefore it will distort your RCA's and speaker wires' signals. So the remote + power wires should be ran on the left side of your care, and the speaker wires + rca's ran on the right side.
Make sure to sand your ground location down to BARE metal. No paint should remain. The more joints/welds the current has to go through to get to the engine black to battery -, the more resistance the ground has. Which is bad.
My ground is under the bolt holding my rear seat down. UNDER the seat metal. The ring is touching the chassis of the car. Which is good.
