We have a low miles, well cared for 08 plate Cooper convertible, completely leak free until a month ago - then without warning the passenger footwell was wet. All very curious, wash the car, no water - but drive off and it appeared from below the door panel, over the plastic trim and into the footwell. Dependant on the slope of the car it could be persuaded to run into the rear footwell as an option!!
I removed the door trim (card) (thanks to others on the Forum and YouTube for guidance!) and with showering hose applied, could now see water penetrating (creeping around) the door seal just below the junction of the two seals at base of the front 'A' pillar (you need a torch!). Slight water runs down the crevice between the vertical seal and the door and collects in the nick along the bottom of the door, between metal and seal - just below the door card. You can't see it with door trim in place. But, move the car after rain/washing and the collected water has momentum to come over the trim to front or back footwells...
Rubber seals look undamaged but would appear no longer pliant enough to apply pressure on front top edge of door (near mirror) to hold back water.
Option 1, replace seals. Dealer wants £150 for seal which does A pillars and roof (all one) and £70 for seal to sides and bottom of door opening (+ VAT, not incl fitting...!) say £400 at least
...thus, swiftly to Option 2 (a bit of a fudge, but somewhat cheaper and WORKED!).
Option 2: Ease the two meeting rubbers off the flanges at the base of the A pillar for a short length, note that the vertical one slots up into sloping one a little bit (15mm or so). Take a short length (about 75 -100mm) of foam pipe insulation. Cut a lengthwise segment sized to fit with an easy push and but stay put, down into the hole of the vertical seal. (The outer curve of the insulation is just about the right profile!). Do the same for the A pillar seal and insert into the hole above the vertical rubber, (not the outside one of the two holes). Refit the seals onto the flanges and hey presto, sufficient force is returned to stop water ingress at that critical junction.
Yeah I know this post will take a lot of stick for bodging, but honestly I was so disgusted at the short life of the seals and high replacement costs that I was determined to find a simple solution for the shorter term perhaps.
The amount of water which pours into the door from the mirror is also ridiculous. With the card off, I found water was hitting a cable within the door and tracking to the inner metal doorskin. From there it found it's way to one of the holes (near to the speaker) and 'bled' through. I sealed the spare holes (on the inside) with waterproof tape and importantly re-bent the wire to drip away from the doorskin - result, no more water through the holes and dry carpets
Tip: to see if this door/seal area is your problem, try placing a short length of wire temporarily over the cill trim and door rubber, such that water which collects in the nick between the door rubber and the metal of the door just under the trim, will be allowed to drain out where the wire is just indenting the rubber a bit. Good luck!!