The obvious cause is of course damp.
I had this problem about a year ago, and eventually I pinpointed the culprit as the main earthing point hidden at one end of the battery, down the slot next to the inner wing. There was corrosion at the bolt holding the battery earth strap, and several other leads. The solution was to undo the bolt, clean up the wing surface down to shiny metal, emery the terminal connectors, and refit using a new bolt smeared with special electrical grease.
The other prime suspect is the fuse box. Water can drip over it from a leaking windscreen seal, corrosion in the drainage channel or door pillar, or damaged paper seals behind the fuse board. It can also splash up from the drainage outlet under the bottom of the wing, if you have been through a puddle.
Try cleaning up all the fuses and using electrical grease. It's worth periodically twiddling the fuses just in case.
Does your car have a fuel pump relay behind the glove box? Damp from a corroded scuttle drain, leaking wiper seal, or windscreen seal can get into the relay and corrode the circuit board.