I have an 81 El Camino with a very small water leak in the interior. Recently I had a cracked windshield changed and that did not seem to make any difference. There is no color to it, like anti-freeze. The water comes from the corner of the heater and drips on to the floor mat. Is there any kind of drain from the air vent ducts where water can escape when it rains? The truck has air conditioning but it does not work right now. Is there a drain for condensation for the air conditioning?

Your heater core is probably bad thats about the only thing that can leak on the inside of your car they dont cost very much i would go buy a new one but that is probably what it is :-D

There is a evaporator case drain port for moisture off the evaporator. You'll find it at the bottom of the evap case, down near the firewall and right side exhaust maniflod. It's should have a rubber cap-like thing with a silt to allow drainage. Check that for blockage.
Even though you say the A/C doesn't work, you may be getting water thru a bad or poorly sealed evap case. If the drain port is plugged, the water will back up and escape where ever it can.
How much water is coming in?
I gotta say, tho... Blue81elcamino is probably right. Are you're sure that it's not coolant, 'cuz it sure sounds like a classic heater core leak.
Double check that core!

Good Luck!

Is there a drain for condensation for the air conditioning?
Read this thread. (http://www.elcaminocentral.com/modules.php?name=Forums&file=viewtopic&t=11048)