Hello.

I used to have a similar prob with my 123 - water leak, front passenger side. After pulling out the carpet I located a rusted-thru area with dozens of small holes in it. Find someone who can cut the rusted metal out and have a metal sheet (zinced) CO2 welded across from inside (it would be best to have it welded from outside beneath the bonnet but there is no good access). After it is welded have it painted, put the carpet back on, scratch all the rust you can off from underneath the bonnet, apply rust inhibitor primer, and have it painted with approximately the same shade of colour. People also put an asphalt-like paint over the inhibitor and then the original shade of paint. The effect of having asphalt paint applied after a primer is that it seals every tine hole or space where water could penetrate and carry on the rusting process. The disadvantage is that the paint is mostly black so have it painted over with an original paint and that is it.