pat mcgroin

From inside of the trunk look through the hole at the back side of the lock.
You will see that it is held in place by a U shaped clip.
With a screw driver slide that clip to the side and the lock will come out.
Be careful not to drop the clip or the spring on the little shaft.
Once the lock is out you can take it to a locksmith who can get the key out.
Since it sounds like the key snapped off in the lock there should be enough of it left for him to make you a new key.
Then just reverse the process to replace it.

I almost forgot..
Dont close the trunk until you do this or it will be much harder.