[Cookeh]

Happens if you don't keep the drain plugs in those two 'bins' clear. They do tend to clog fairly quickly if you're using an estate as an estate, and that then stops water/condensate draining. Leave it long enough and, as you've noticed, it will rust through.

Keeping drain plugs/ports clear should really be done as part of your fortnightly maintenance (or done whenever you wash the car, or any time you check the oil/fluids/etc).