1973 450SL Heater troubles (tricky one)

Alright, here is a stumper for all of you. I have a '73 450SL, and whenever I open the windows, heat blows through the lower vents. The car was my grandfather's until recently, and he said that there was a problem with the heater, and that some connections were left disconnected instead of making a costly repair (sorry I'm not sure what exactly was done). As a result of this the heat is either on or off, no in-between. I assume some kind of flap is being pushed open by the change in pressure when I open the windows, allowing hot air to flow, but I can't for the life of me figure out where to start. This only happens on the lower foot well vents, and the "leaking air" stops the second I roll up the window. Help would be appreciated, since I have to make a four hour drive later tonight, my A/C broke two days ago, and we are experiencing record high temps in FLA. Thanks,

Scott

edit: FIXED--I took a drive to pinpoint the leak, and it was not the vent, so I figured it must be a bad seal on the firewall, and sure enough, air was flowing over the hot engine, and in through the 2" hole where the speedo (i think) cable enters the dash. Just reset the rubber seal, and things were fine.