just a quick note, when I got my SL it did the same thing, the washer to the headlights kept runnning, I found that after I checked them out, the wiper arms were very loose, I adjusted them and tightened them down, I have not had a problem since. I don't know how the loose wiper arm effected the water flow, but for some reason it did. Give that a check.
Good Luck
Brian