The problem was solved at the second attempt on my Focus. The problem is
the sealing around the pollen filter below the windscreen on the
passenger's side.

Are you shore it's not a mini :)

See Ford USA TSP below:

My local Ford dealer ignored that complicated procedure and simply inserted
a piece of plastic sign material between the cowl cover and the air intake
box (that holds the pollen filter).  That solved the problem on both of my
Foci.  The plastic was cut slightly long so it folded in the middle,
tent-like; air could get in on the sides, but water from above was stopped.
This has held up for 3 1/2 years on my first Focus.  I was a little ticked
when I discovered what they had done, but I got over it because it worked.
I suspect  the dealer made a nice profit from Ford on this "repair".

     03-18-02 a.. BODY - WATER LEAK - CLIMATE CONTROL - WATER IN HEATER
BLOWER MOTOR CASE
       b.. ELECTRICAL - CLIMATE CONTROL - WATER IN HEATER BLOWER MOTOR CASE
FORD: 2000-2003 FOCUS

ISSUE:
Some vehicles may exhibit a difficult to diagnose or difficult to repair
water leak condition in the front passenger floor area. This may be caused
by sealer skips, loose grommets, mis-positioned seals, etc.

ACTION:
Some common water leak locations and repair recommendations are listed below
to help reduce repair time and increase repair effectiveness.

SERVICE PROCEDURE
Using a high-pressure water hose or a pressure washer, run water over the
roof onto the windshield being sure to direct water over the roof ditch
mouldings, and down onto the cowl (Figure 1). From the side of the car with
the hood open, direct water into the base of the A-pillar in the hood hinge
area (Figure 2). With the right inner fender splash shield removed, direct
water into the back of the right front wheel well area (Figure 3).

After Water testing is complete, inspect the following areas for visible
signs of leaks.

     NOTE: IT MAY BE NECESSARY TO REMOVE THE INSTRUMENT PANEL TO FULLY
ACCESS AND REPAIR SOME LEAK SOURCES. FOLLOW WORKSHOP MANUAL SECTION 501-12
IF INSTRUMENT PANEL REMOVAL IS NECESSARY.

If Water Is Visible At Blower Motor Connector, Blower Motor Resistor, Blower
Motor Mounting Flange, Or Foot Air Duct:
The most likely source of the leak is the fresh air intake. (Water entry
into the air inlet housing may be visible by looking through the
recirculation door as it is opened or closed. To do this, remove the glove
box, turn the ignition key to the on position and push the recirculation
button.)

Inspect for the following:

 1.. Inspect for visible gaps between the windshield and the cowl grille
(Figure 4). If gaps are present remove and replace the cowl grille,
(1S4Z-5402228-CA) per Workshop Manual Section 501-02.
 2.. Remove cowl panel, inspect for gaps between pollen filter housing
gasket and sheet metal plenum. If present, inspect for studs bent or off
center. If off center, use a drill to slot the housing (Figure 5).
 3.. Inspect for bent plenum sheet metal flanges - repair as necessary.
 4.. Inspect Pollen Filter Housing gasket for damage. If damaged, replace
Pollen filter housing (YS4Z-19N619-EA).
If Water Is Visible On Evaporator Case Seal At The Heater Core Inlet/Outlet
(Figure 6):
 1.. Inspect for a mis-positioned evaporator case seal at the heater core
inlet/outlet. Repair as necessary.
 2.. Inspect for loose evaporator case housing nuts. Repair as necessary.
If Water Is Visible In The Right Kick Panel/A-Post Area:
 1.. Inspect for sealer skip behind windshield roof ditch mouldings. Fold
the ditch moulding back and inspect for pinhole in the body seal just above
the windshield. Reseal as necessary (Figure 7).
 2.. Inspect for wire harness body grommet loose or mis-positioned.
Reposition or reinstall grommet as necessary (Figure 8).
 3.. Skip in windshield urethane. Reseal as necessary.
 4.. Inspect for sealer skip in A-post to cowl joint. Reseal as necessary
(Figure 9).
 5.. Inspect for sealer skips between the cowl top to bulk head, just above
the main harness grommet. (Figure 10).
 Rerun water test after repairs, to verify all possible leak sources are
sealed.

     PART NUMBER PART NAME
     YS4Z-19N619-EA Pollen Filter Housing
     1S4Z-5402228-CA Cowl Grille
     F7AZ-19554-CA Clear Silicone Rubber

OTHER APPLICABLE ARTICLES:
01-9-2,  02-15-6

SUPERSEDES:
01-9-2

WARRANTY STATUS:
INFORMATION ONLY

OASIS CODES:
110000, 111000