Does it work from the door switch. If not replace the actuator inside the door or look for a broken wire in your door harness. I have seen them in the rubber tube that runs from the A pillar to the door.
No harder than making a long bed a short bed except your adding frame instead of removing it.
Would be a lot easier if you could find the rear section of a long bed frame to graft on. Just cut off the short bed frame and replace with the long bed.