I posted a similar question a few months ago, after living with the problem for years. I thought I had tried everything there was to try. I experienced this with two different 123 sedans.
Someone told me to clean out the drains below the hood hinges, that's right the hood hinges. There are drains there that drain under a brace toward the engine compartment. Then there are drains just inside that brace, between the firewall and the wheelwells. I used a piece of bailing wire (for city folks, I'll bet that a coat hanger will substitute for bailing wire) to clean out both these drains.
I have had some very hard rains since, and have had no more problems.
I know it sounds bizzarre, but it worked for me. I need to search back, find the post and thank the person that answered.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in