had this before and it may be you problem aswell and took me ages to find it if you look down in between the front of the fuse box and the black metal firewall in front of it.in the corner you will see a small rubber flap about a inch square or less which acts as kind of air pressure relief flap, make sure its not stuck half cought in its opening,if it is it can let water drain down onto the loom rubber bung below it going through the bulkhead,and if the rubber bung has extra wires or boost gauge hoses going through it,it can cause water to leak in and you wont see it because its behind the inside bulkhead matting,you could put some extra sealant around the bung and try it out,but you will need to pull the inside matting away to be sure its comming from there or cured