Its often been found to be a front heater matrix leaking into the air box and then out to the drivers side see these search results https://www.google.co.uk/search?q=wet+drivers+foot+well+mazda+bongo
Getting to the front heater matrix is covered here but it can be quite fiddly and you will need a ...

Ps I have a 2.0 litre petrol bongo

Hello, we’re in summer hols in France , head back home this weekend about 800 miles to do .
Our water levels dropped on the way down here significantly , I’ve been keeping it topped up but there is some sort of slow leak . I’ve just noticed it’s quite wet under the floor mat on driver side too.

I ...