There is another batt box under the driver seat with some kind of relay in it that could be moved for another batt.
I looked ther when at the dealer, it is more than a relay.
Looks like a control module of some sort.
lot of wasted space arround it.
Would hate to cross a stream and get that well full of water.
Would be great if someone was able to make an extention cable and move it under the back seat.
I am 600km from the dealer cant run down and look at things.
when I get mine maybe June will look at doing some thing.
My WG/WJ has the second battery in the back in a custom made enclosure with the sub woofer.
Wife will not let me do that to the new one.
landcrusers here come with a space for second battery already.