XJ-12:
Under the hood, you will see a power point to hook a charger or jumper battery to, the terminal to boost the car, is towards the top of the bonnet (hood), beside the engine, on the mid right side, mounted near the windshield (that means, if its in north america, im saying its closer to the passengers side than drivers side. This will light the interior up enough for the trunk release/lock motor to open up.
One report says there is a little key slot on the trunk lid, use the ignition key and twist a bit hard, and trunk will open.