The spider is essentialy a relay, 4 wires in each side, hence spider. If this goes wrong you can wire other wires into the working ones. So say 3 outputs work, put the forth onto one of the working ones. This is how we cured the intermittent non start issue on our 300tdi, the wire that was not being properly switched was the one for the starter... Problem solved.
It is not likly that rain would cause the spider to cause this, although its a landrover, water could be getting in anywhere I guess.