My experience was to run a water test : generously spray water around the car and then check for leaks into the cabin from any possible gaps, i.e. door, rubber linings, etc.
I did the same for the 300se when the back was almost becoming a swimming pool only to discover that the rear light covers were faulty as water was running through it. My assumption that it was going through the rubber linings were naturally unfounded.
My dad's 260e had water leaks coming in from the rear window sill - which I believe to be the cause of the rubber lining. I can't fix that - so the expert would have to take the rear glass down and reseal the rubber (without having to crack the glass).
Best bet is to look for any possible water marks.
Do let us know of your progress.
ps. The Chinese culture believe that water flowing into your car may indicate good fortune coming in!