Check the switch on the trunk lock (it's on the lock on the deck lid by the release cable). Make sure it has free movement, and make sure there is nothing in the way on the latch where the trunk locks into. If it still stays on, I suggest using an ohm meter to check if it shows open and short when it should. If you still have problems, you probably do have an open or loose wire somewhere. But I would just play with the switch and I doubt its anything more than that.

I do suggest if you keep seeing the lamp on your dash to remove the bulb in the trunk light untill you get it fixed. If the lamp on your dash is on, the trunk light is on, and it will stay on after you shut the car off.