you can unscrew that panel off the trunk lid from the inside fairly easy. As long as you can fit, fold the rear seats down and use a flash light. Once that panel is off, everything mechanical will be in front of you. I recently had to take all the bits out of my trunk lid due to an accident. The job is fairly simple, but the fact the the trunk is shut will complicate things a little more.
Once that panel is off, you can see the lever that links the locking pump to the trunk latch below. I'm not a hundred percent positive, but I'm sure you could fiddle with it to get it to open. Then from there, you'll already have the pump exposed, you can remove and replace it.