Hi there, just noticed your post and thought I would offer up my 2 cents. I have lots of experience with this stuff and I do electrical for a living.
Myrefuge made an excellent suggestion, I had the same problem on my toyota van, and putting a belly pan helped out alot
I'm not sure if they make them anymore, but a cover for the distributor to protect against light splashes would also help out. The most likely culprit in alot of cars are splashes and moisture in/on the distributor
Lastly, this sounds abit anal, but whenever you have an electrical connector that may be exposed to water or weather, I like to treat it with Penetrox or similar product. It is a grease that protects against corrosion for electrical parts. You don't need to open all your connectors and apply it, but if you happen to be doing a repair and come across a connector that needs it, squirt some in, you'll thank yourself the next time you need to undo a connector and it isn't corroded to dust.
Hope this helps