Just did a quick visual inspection on my 240.
Once i removed the small rectangular carpet (where back passenger rest their feet-right side) ,i've noticed the bottom floor was wet, no pudddle just soaked wet carpet.Other side is fine.Looked under the car,no apparent rust holes...is this a common water leak spot or plain condensation?
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jlp

Could be leaking in under the dash. Take the passenger seat out, pull the carpet back, lie down in there with your head under the dash. Use a flashlight and do a thorough inspection while someone sprays the car down.

On my 81 242 the right rear floor was soaked (and rusted)from water leaking in past the seat belt anchor mounted on the side of the car because of blocked rocker panel drain holes. Make sure all four (four on each side of car on the 1981) of your rocker panel drain holes are clear. Just get a small knife blade or popcicle stick and push it into each hole a few times. They are on the bottom seam of the rocker panels. You can feel them or lie down and see them.

Good luck.
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Thanks everyone for all the information and advice, Doug C. 81 242 Brick Off Blocks, stock, B21F, M46; 86 244, B230, 140k , auto.

If it were summer and you were using the A/C a lot, I'd say that the A/C evaporator drain was plugged (you should periodically check this anyway). Since it's winter, more likely candidates would be those mentioned by "Hear to Tempt You" as well as windshield wiper motor shaft seal and the seal around the base of windshield itself. Does the car have a sunroof? Try the old garden hose test, where you have an accomplice spray the outside of the car with a hose while you contort yourself into a position where you can look for leaks under/behind the dash.
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Mike W., '79 242GT (project car), '85 245DL (daily driver 1), '87 245 DL (daily driver 2), '90 244DL (son's project car), '91 744T (daily driver 3), '92 245DL (parts car), '94 944T (wifemobile), Largo, FL

Water is probably leaking in from the front somewhere, and getting trapped where you feel it. That's the lowest part on the car, so water will collect there.

I suggest you look for your leak and seal it. Check your antenna, and hood release cable. Those are common leak spots.

Good luck,
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If you listen to the radio in Portland, OR, you may know me as 'Portland's Favorite Soul Brother!'

I say train the dog.

Does this year have drain hoses from the trunk lip under the back window through the trunk? If so the passenger side one could have been knocked off when overloading the trunk with flats of beer.