On the AMG CL version (yours must be similar) there are 4 drains which can get blocked in the bulkhead area causing water ingress. On the drivers side there is a drain to the bonnet grille, with a large star shaped bung visible when you open the bonnet. Open up the bung, you'll be able to feel if blocked. In the middle of the windscreen scuttle between the wipers there are a row of drain holes - these seem to get blocked easily from debris from the screen. A leaf can block them also. Make sure these holes are clear.
On the passenger side you need to look into the plastic grille on the bulkhead. If full of water you need to unclip and remove the grille, there is a rubber drain hole at the bottom, which you need to poke open with something like a skewer to drain the water. However,the worst culprit for blocking is the drain adjacent to the passenger side bonnet hinge. Follow the wiper scuttle to the passenger side- there are 4 square holes which also block check these are clear, wipe away debris. These 4 square holes drain water from the scuttle into a plastic open drain fitting right by the bonnet hinge. This is the problem area, because the drain tube has a 90 degree bend about an inch or so from the opening and blocks easily. You will need to poke something flexible down this to clear it. I used curtain wire. Use a large syringe or something similar to flush.