Check the trunk lid opener button in your dash(one of those four in the middle behind the gear stick).Might be stuck.
If not try spraying contact cleaner in the light switch and in the trunk lid opener button.
I use isopropilic alcohol spray.
You can get it in any electronics parts shop,or some hardware stores must sell it.
If none of these will fix the problem,then you have a short in the wiring harness somewhere,which will not be easy to spot...
Hope this helps