Progress has been made! I have released the glovebox, and located one switched live and one permanent live fuse.
It's easier, or less difficult, to release the glovebox after driving the car with the heater on, as this makes the clips slightly more compliant. They still don't give up without a fight, though.
I'll resume tomorrow, because I'm going to make an earth connection by drilling the strengthening strut at the end of the dashboard, having removed the trim panel (carefully, because it has the switch for disabling the passenger airbag mounted to it), but my rechargeable drill needed to be charged. The mounting bolt for the fuse box would suffice as an earth point, but it's not easily accessible.
Tomorrow, then, I'll make the earth connection, route the cables (I may leave the rear camera for now) and tidy up the excess wiring, the plan being to strap it up using tie-wraps.
More later.