jassz
I was visiting a local shop last winter and he had a 300D in there and could not find the water leak. I showed him all the usual suspects. He cleared all the drains and it still leaked right down the firewall in front of the driver. Returned it to the owner. Got it back two weeks later and it was still pouring water inside.
Then I was out and he was scratching his head and saying some bad words. Suddenly the lights come on and there was the problem.
There is a large black plug on the firewall that the wiring harness and some vacuum lines go through. It was a little out of place. So we pushed it back in. Then water tested again. No leaks. I called him this morning and he said zero leaks with that car since.
Just one more thing to check.