My truck won't start sometimes when it rains heavily?

Hello,

This issue could be due to moisture entering the distributor or the coil. If the seal around the cap is broken moisture can form inside the cap. This will cause it not to produce any spark and the engine will not start.
As it dries you are able to start it, because it is producing spark again.

Check inside the distributor cap for any sign of moisture and corrosion. Typically, it will have white residue if moisture has entered the cap.

I have attached diagrams for your reference.

Please let me know if you have any questions.
Thank you.