It pretty much boils down to whether it is the switch or the fan motor. The first thing to do with the vent fan is to check if the fan will spin in the reverse direction just by giving the blades a spin, and then check the switch and see if it is supplying power to the fan in reverse. My guess would be that the switch...isn't.
Some couches have backs that come off to make them easier to move. You didn't say if it was a sleeper sofa, or just a plain old sofa.
Howard and Peggy
"Don't Panic"