This might sound stupid but here goes,Sold my daily driver 83,purchased a nice 86,58,000 miles.It had been sitting for a while,now im using it every day.Its been raining here A LOT! Im driving,need to ''STEP ON IT'' to get out into traffic and out from under the dash (black plastic behind radio)Heater duct I believe.....THE GREAT FLOOD!! What gives??? Not coming from the windshield either,Ijust had a brand new one installed.

Sounds like a heater core to me. Not very expensive, but not a whole lot of fun to change out. But not bad.
Good luck

You may have a clogged heater box drain. It's a little rubber flap in the bottom of the box. If it's blocked the rainwater will accumulate inside the box. It's pretty much hidden from view unless you remove the plastic inner fender well. If you remove the blower motor and get a flashlight down inside you can check it.

Here is what it looks like from the outside on an AC box.

http://elcaminocentral.com/users/elcamino/pics/door_op.jpg

This is what it looks like on a non-AC box.

http://elcaminocentral.com/users/elcamino/pics/door_op_non_ac.jpg

another floridian here with the same problem. but i got it on both sides, pass, and driver.

Thanks guys! Its Fresh (well as fresh as rainwater can be) water .I'll check the drain.(Its an a/c car) The sun is out here,for the moment :)