If the key is in the position that would allow the lock to turn, maybe with a screwdriver you can turn the lock cylinder back to the position where the power lock will operate again.

A screwdriver blade should be able to take a little more torque than a worn out key, so hopefully that will work, you'll be able to then open the trunk and use it normally.

I would first try to get the key piece out, as mentioned with a lockpick set or other tiny screwdrivers- jewelers screwdrivers may do it.

If you can't get it out but are successful opening the trunk otherwise, the lock assembly can be removed and unclipped from pins, and a replacement from a junkyard car put in. Not too hard of a job, just fiddly.

Good luck!
--
89 244GL Turbo // 92 244 M47 \\ 90 745T // 76 242 Convertible

I was able to 'pick' the broken of key out of the lock with a little set of four awl/picks sold at the big box hardware stores. Using one above and one below was able to wiggle it out.

http://www.harborfreight.com/4-piece-pick-and-hook-set-66836.html