Morning Ian
If the car has been in a crash it may be that the fused link at the battery has gone which ensures that all power is severed, so all circuits are dead. Can you get into the boot from inside the car? I would look at www.newtis.info and check, it is a technical information site the same as used by Dealers. As my car is an X5 it is essentially an estate so I just drop the back seats, I do remeber reading something about trunk opening (so obviously an American site. Found it
a) Take out the two torx screws just below the glovebox on the left and right to remove the panel below the glovebox. Disconnect the power socket and the light which are attached to the panel.
b) Below the fusebox there is a big flat horizontal junction box (Junction Box A4010). You can spot it easily because there are two big connectors going into it facing you. There is a flat wide connector plugging into the BACKSIDE of this same junction box. There are about a dozen small wires going into it and one big brown ground. This is the one you need to get to. (Connector X14270) Ideally, you'd want to disconnect this, but I could not manage to get it out from that position.
c) Identify the small wire which is gray with a stripe of green (it looks mostly gray). If you get the connector out, this is pin 12. It's the only grey/green wire in the bunch though.
d) You need to very briefly apply 12 volts to this lead to pop the trunk. I did it the ugly way - I very carefully razored off some of the insulation to expose it. Then I just attached some leads to my new battery. I stuck the ground lead into brown wire connector for the power socket I had disconnected when I removed the glove box panel. Then I just brushed the positive lead against the exposed pin 12 wire. The trunk popped instantly.
I hope this is some use apparently he had taken out his battery and closed the boot (Trunk)
Dave