how can I manually unfold the bed in a '92 gmc vandura?

Have you replaced the fuse yet?

If you hear any electrical "clicks" when you press the switch, you probably have rust built up on the rails beneath the seats. Look underneath from the back. Some fine sandpaper or steel wool will work wonders. You may have to remove the rails to achieve the best results. when finished, a light coating of grease to prevent future rust should put your seats back in business!