Check to see that the drains near the hood hinges are thorouhgly cleared.

When I picked up my 300D (diff chassis obviously, but similar drainage designs) it leaked a good amount of water into the cabin (ends up in the back seat). I cleared all thr drains (removed the battery) and its fine now.

Use a hose and check that all the drains function properly, including the sunroof. To see where the sunroof drains, open it and run some water into the tracks to simulate heavy rain.

__________________
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E