Can’t remember what it’s for on the standard car, but somewhere in the passenger footwell, up high towards the base of the windscreen and outboard there’s an open grommet that is traditionally used for aftermarket amps.
However, battery in boot, bulkhead adapter, or walking everywhere are all things I’d rather do than try and squeeze a decent gauge cable through that grommet.
There’s a big hole in the bulkhead behind the wiper stalk for the AC inlet. I think that’s got a big grommet on it for AC and the dealer-fit alarm wiring too that might be useable?
edit: bonnet release cable grommet. But you will 100% regret your decision to not drill a hole.