its usually the scuttle area but anyway heres a daft idea. soak up as much as you can with towels sponges wet vac etc until you can dry it out properly ,carpet up job etc. however you choose.soak up some with white kitchen towels to make sure you are looking at clear rainwater rather than antifreeze although you would know if it was that anyway.now heres the detective bit.mix up a bucket of coloured water.use whatever you wish to make this.a small amount of coloured emulsion paint,food dye of even beetroot juice whatever .gently pour this over the passenger side of the scuttle area/windscreen corner so it flows down and drains out of the. bottom.hopefully!. then check inside again to see if any fresh but now coloured water has appeared. again white kitchen towels are best for this.if so then you now know that its definately somewhere in that area.check usual drain bung,seams,etc. .