Kev,
Have been looking through the Nissan, Ford 4x4 club and found this...
Bottom of the wings, a drian hole gets blocked with mud build up, open the dor and look at the gap between the wing and door pillar below bottom hinge area (both sides). Squirt a hose through there to remove this muck.
Below windscreen there is a plastic grill type panel on the outside. Remove this panel and at the passenger side there is an inspection panel. Remove panel and seal with an automotive silicone type sealer.
my 2001 terrano has same problem, mine was water ingress through the pop rivits of the galvinised vin plate.sealed mine and no water in footwell after all that heavy rain!
Check the seal on the bulkhead for the heater as well.
hi i had a problem with my 1994 terrano footwells wet it turned out to be a cover plate that is above the heater/fan air intake.it has a foam seal around it and i found mine to be distorted.water then dripped straight down into the heater fan and it was blown out of the ducts in the footwells.once sealed all ok.hope this may help in some way?
Hope these suggestions help.