Agree with this article.  I had an 1986 745TGO - and when it was 9
months old the saga of fixing the leak began (under warranty
thankfully).  Took several visits to the dealer over about a year
before they finally realized the problem.

Water Leaks. I too had wet floors and followed the last advice on the
topic by checking the plastic vent covers at the sides of the
footwells in the front seating areas. These were fine, so after
careful observation and lots of water I found the cause of my grief.
The wiring harness going from the body to the doors (electric windows,
speakers etc.) has rubber protective sleeve. This had split open and
since the entry point into the car is several inches lower than that
of the door, and once it enters the car it no longer has the this
cover, the split acted like an open funnel, and guiding water in. Open
your doors and check the cover. Duct tape worked like a charm. Now I
just need to dry out the carpets and rubber under-pad.
[More:] I assumed the 740 had a windshield gasket leaking problem and
I had the windshield reset. That did not stop the water leak and wet
carpet in the 740. It turns out that it is leaking, as others have
reported, from the unused left air intake behind the panel at the
driver's foot. To discover it I ran a great volume of water through
the air intake beneath the windshield and sure enough, after 10
minutes a small, but steady trickle of water was coming in.
Unfortunately this has rusted the floor through to the outside.
WARNING, if you have wet carpets, get it fixed, the floors will rust
through in a very short period of time. The first thing that my
mechanic attempted and fixed on mine was the air intake seal at the
left bottom driver's foot well. It supposedly resides behind a small
plastic panel which has an air grille on it. [Contrary note:] You may
find a problem in this area, but the seal on the panel was not the
problem. There's water getting in there, probably from the seal around
the windshield, that's causing the problem. I'll bet if you pull the
trim up, you'll find rust. Ever replace the windshield?

[Another Query:] How do I prevent water leaks in our 1986 740 GLE
wagon, especially after moving to a house with a sloped driveway. The
problem is water leaking into the interior in the front footwell
areas. The problem only occurred when the car was parked facing
uphill. It turned out that  runoff from the compartment housing the
fresh air inlet to the heater and the windshield wiper mechanism was
directed into the car by a plastic tab on the  plastic plates used to
close off the footwell airvent openings.(Older models like my '69 164
and, I assume, contemporary European 740's without A/C have  fresh air
vents in the sides of the front footwells.) After removing the plastic
tab in the blanking off plates, the water leak stopped.   [Response:]
Replace the cap for the fresh air intake with p/n 6848804; it comes
with a new seal.  It is a common problem. Break the old one out and
pop the new one in. It should click in.

[Tip from Zee]  For tips on locating and fixing windshield leaks, see
http://www.7net7.com/glasnet/leaks.htm