Thick wire from Alt, connector at starter, red wire up to black box, and battery positive are all one and the same. They all join up with one another no matter where they're routed. The red wire to black box can be taken from either the B+ on alternator or at starter, it doesn't matter, they are in effect all one connection:
I suspect that the way its wired has as much to do with vehicle assembly at the factory as anything. When I replaced all my original charging/starting cable I wired it exactly as above, despite the way VW had routed it all. I just fitted an even thicker cable than stock between battery and starter (see WIKI link I posted earlier) and finally ran a feed up to black box in the engine bay, from starter connection.
This illustration is (as some will recognise) from the VW Beetle fitted with alternator but makes it easy to understand. The wire colour coding from bug to T25 has not changed and even now I keep a laminated copy of the later beetle diagram as it makes wire tracing a breeze on the T25 - good tip if you struggle with the Haynes version.
