can't offer any useful advice, had the same problem with my spider, spent hours checking it out,had power to the wiper motor but they just wouldn't work, so I bought a used wiper motor, went out to the garage, placed the used wiper motor on the bench, hit the wiper switch one last time and it worked, the used unit is still on a shelf in the garage and the wipers have never given me another problem, and I'm not asking why, This is my 10th Italian sports car and I stopped asking why along time ago.
BUT has the car been sitting long and do the contacts need cleaned?