Blocked front water drain vents are your most likely problem. The wheel arch drains, screen drains or outlets to the wiper drain must be the areas to concentrate on. If water is coming in on the screen you would see something around the dash. The drain offs go down through to the bottom of the arch so it is worth looking around the plastic cowling on the wheel arch to see if there is anything there which looks out of place.
There is also a drain under the plastic cover where the ECU and pollen filter is. If it is blocked water builds and spills in through the pollen filter, into the heater and usually soaks the carpet on the passenger side.
If you have a sunroof it also has drain offs. On the 260 the pipes go down the A pillar to the door side of the arch. On mostly mk1's but some 2's up to about VIN 400 the tubes may have been too short allowing water from the sunroof to soak the body control unit (and others as I found out) giving major problem with electrical signals shooting all over the place and flood the front floor carpets. The answer here is to replace with longer tubes and treading though making sure they send the water to the ground.