I agree that your water drain may be plugged (this is the drain that allows water from a rain storm or car wash from overflowing into the ECU and other vitals).
This is a poor BMW design flaw (very well documented for the e36 series - there is a nice article at pelicanparts.com that details this issue) as the plug can easily be overwhelmed by large volumes of water and the water will leak into the passenger side footwell.
This happen to me right after a car wash on my 93 318is and my ECU got flooded (I was furious
![Mad [mad] [mad]](/images/smilies/mad.png)
as I was stuck in the middle of a car wash bay and had to wait 2 hours for a tow).
Luckily after drying out the ECU for two days the car started up and I luckily avoided a $600 hit for a new ECU.
You might want to avoid any car washes in the future and check to see if the drainage has a bunch of leaves or other crud built up.