>Can anyone tell me from memory the routing of the
big + wire that comes off the battery?

See page 97.8 through 97.9a of your Bentley.

You could also connect your light wire onto the big terminal of the
starter or alternator if you don't want to come into the battery box
from underneath the car (though you would need a long heavy wire, and
fuse it close to the power source, not the lights!)

However, it's probably easiest to come up through the battery box and
piggyback onto the battery + cable or connect at the fuse/relay
panel, Terminal 30.