I was at one the other day and one of the stalls had an actual pick axe, which I was very tempted by, but I didn't want to have to carry it back and didn't want to attempt to explain Minecraft to my family nor any police who would have inevitably pulled me over and searched my car. :C
Is it a hole that's supposed to be there or a rust hole ? It could be a drain hole which if it's letting water in means it's blocked somewhere, siliconing it will make it worse (well it won't make it betterrer).
Get a long piece of wire and have a poke around in it, see if you can blast some water into it under pressure and if you can find where the exit of the drain hole is and do the wire poke in there. The exit is where the blockage is most likely.
Found this which seems to cover most leaks on a Mondeo ;
http://www.mirez.co.uk/WaterProbs.htm