The passenger side floor is wet, right under the vent and in the heater core area. Most of the wetness seems to be to the left side. Im pretty sure its not the heater core because its only wet after its been rainy. Its been wet on the driver side as well but it seems like after I closed the vent its been fine. I looked on the underside of the piece of carpet Ive got on the floor and could see an outline of where it was wet. Could it be the seal around the windshield? Can the water be coming down the air vent somehow?

Here is a picture, Ive shaded the wet areas in blue.



http://img132.imageshack.us/img132/4567/wetjq2.jpg

If the leak is coming from the core, I would guess that  you should be able to smell antifreeze.

I have a 73 Runabout that sat outdoors for too long and the vent sheet metal, (below the windshield, under the louvers), rusted and that is where my leak came from.

 Located on the passenger side, the heater assembly attaches to it with a soft rubber doughnut between heater and body.

I welded as much sheet metal patch as I could to fix the problem. Only trouble is there was not a lot to weld to. Will use bondo to fill in the rest

Al

I agree with "aPintoNut", this is the first place I would look.

I noticed that my 73 Pinto was leaking water half way down the window seal on the passenger side of the interior.   The water then dripped down on the door sill area, then to the floor.   I was lucky because I do not have carpeting.   My guess is that my leak was further up the window seal and the water followed it's way down.  
My car is not rusty and the rubber felt good, so I don't know why it was leaking.  I treat my car as a low cost driver and not as a classic.  So all I did was buy some some flowable silcone windshield & glass sealer and got it to flow behind the metal trim.  So far this bandaid repair has worked.