Where is the other end of the 12v battery positive cable?

I'm working on a friend's 2011 and it acts like the battery is dead when it isn't. 12.6v at the battery terminals in the trunk, less than 6v at the "jump start" points under the hood. Car can't start, no interior lights, key fob does nothing, dash "door ajar" light is faintly visible when door is open, electric door locks faintly try and fail to lock/unlock from door button.
So there's a poor/broken connection somewhere and it's clearly not the negative as that is a 1ft cable straight to the body, already tested good.
So, where does the other end of the 12v positive cable lead off too? It disappears under the rear passenger seat and is gone from there as far as I can see without further dismantling this car.
Yes the battery is charged, yes the terminals are correct, yes the terminals are tight, yes the fuses are good, no there isn't any corrosion, etc etc.
(Car starts and runs fine with another car connected to the jump start points under the hood, I don't dare try letting it run with jumpers removed after starting as that would be equal to running a car without a battery, bad stuff.)