I sold my last W126 about 6 years ago, so I'm going from memory here....

I believe the small connector is the power/ground/etc connector. The larger one has the wires that go to door latch switches, trunk switch, etc. If you pull the small connector, you disable the entire system. No need to worry about the big connector as it just carries signals to/from the various locations.

Regarding the leak, has your windshield ever been replaced? A lot of times the seals leak (either through improper installation, or someone using an aftermarket rubber seal). The water then leaks into the car through the failed seal.

I would try taping a bunch of paper towels as high up the passenger floorboard as you can (get them right up against the metal) and then shut the car and hit the edges of the windshield with a garden hose on the highest 'jet' setting. Go all around the windshield edges, then open the car and see if there is any water on the paper towel.

It could be a battery leak, but it sounds like you have too much water for that.. Your battery would be too empty to start the car if it had leaked that much, in my opinion.

__________________
'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)