We have the same thing and ours are operated by "rocker" switches. You push the top of the rocker switch for low or high and push the lower part of the rocker switch down for the other speed, however yours are set up. I think ours are set for high is on the top and low on the bottom. As you mentioned there should be 2 rocker switches one for each side fan. If the fuses are good they should work, or perhaps a wire is loose on the fan itself; but that seems a long shot.
I have a feeling you already know this information.... 8)
Marsha~