We had this problem but it was not condensation it was a leak through the scuttle via the bonnet pull cable grommet which sits almost directly above the fusebox, it soaked into some foam which backed the undertray as well. Could also be the ignition switch, try jumping the starter straight off the battery while the ignition is on. Also check the main +ve feed to the back of the starter, mine had almost corroded through and was a bu**er to turn over for months, eventually it flashed and burned right through, once I'd replaced the cable I never had the problem again.