Originally Posted by steve2006

The picture below shows the wiring loom running under the trim on a RHD LS400 so bear in mind this may be on the other side on a LHD model.
The trim is just clipped so is easy to remove to access the loom. Remove a section of the black tape insulation to expose the wires within the loom.
Locate the thinner of the plain black wires then cut the wire.Trim the insulation back on the black wire nearest the trunk then extend the wire with another suitable piece of wire long enough to reach the battery positive terminal.
Briefly touching the other end of the wire to the battery positive will activate the trunk lock solenoid and the trunk will then pop open.
Once you have opened the trunk remove the temporary wire extension and rejoin the two ends of the black wire preferably by soldering and heat shrinking insulation, rewrap the loom and refit the trim.
It sounds complicated but should take no more than 15 minutes from start to finish.
The reason why the wire is cut first is to avoid any possible damage to the control unit by feeding 12 volts into it the wrong way.