How about removing the switch and hitting it (lightly) with the blowgun for 15-20 seconds. That ought to dry things out and not leave a nasty, oily residue like the WD-40 (throw that stuff away). If you are feeling really ambitious, take the switch apart and clean it with rubbing alcohol and q-tips. Re-lube with dielectric grease and reassemble. The grease will help displace the moisture next time this happens as well.

Jay.

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On the road, currently:
'83 300D (German spec.) 168k mi. - - Wolfgang (tucked snugly away for the winter!)
'87 300D 375k mi. - finally went over to "The Dark Side"
'87 300TD 225k mi.
Cannondale 66cm CAAD5 w/Campy/Jamis Aurora
Hujsak Custom Fixie/Landshark Track Shark Custom/Ahearne Custom 29'er