It would have been more helpfull if you said what they did in the garages and what tests they carried out.

So is it using coolant do you top it up?

Does water come in only when it rains or you wash it?

Look on the bulkhead where the wipers are there will be drain holes these get blocked and the water swashes about and finds it's way down into the carpet.

And then get under the steering wheel area and have your mate with water pour over screen area can you see any leaks? And same on passenger sides.

Check the inside edges of the door when its been raining, I have known water get past the seals or even under the tread plate if it has one.