Hello harajack and another welcome to the club.
I had a forum article going on water in passenger footwell, this may give you some clues if your motor is anything like my model. I think it's worth looking it up.
The little plastic "rivets" are like a rawlplug with a plastic insert instead of a screw. If you dig around with a couple of little screwdrivers and prize this centre core out the outer bit will contract and can be extracted; these are the bits which hold the grill in situ along with some self tappers on the front edge. Watch out though, the little bits fly out quite suddenly and easily get lost. I lost all mine and now use zinc coated self tappers instead.
The wipers come off by pulling the hinged flap/cover back and removing the nut from the wiper arm spindle. The arm will probably be seized onto the splines but a soaking with releasing oil or WD40 (leave it for 10 mins or so), then two large screwdrivers to prize the arm carefully off.
The vent grill is not (normally) stuck to the glass and doesn't need to be.You should be able to lift it off when the wipers are off.
Under the grill and on the passenger side there is an oblongish plate held by four self tappers. This plate is over, but not blanking, the aperture which allows fresh air into the heater. The oblong plate on mine was very poorly sealed and was a path for SOME of the rainwater to drip into the air intake. A dose of sealant sorted that out.
However, as I say in my article, I have left my carpet and sound proofing loose because water still gets in, in small quantities, from a few places and I am still slowly working around to find them. I know that my sunroof and front scuttle drains are working at least fairly well but not yet sure that there isn't some partial blockage behind the front mud flaps.
I shall either succeed or instal a bilge pump.
Good luck with yours; would be happy to know what you find.
Charles.
