The base of the heater channel has almost completely rotted away, showing the actual channel which carries the hot air to the front of the passenger compartment. Note the drain hole; moisture condenses out of the warm air from the heat exchangers when it passes down the heater channels. If you block up the drain hole, rusting will be rapid! New heater channel/sill assembly base repair panels are available, but be wary of buying a car repaired in this manner. The hidden heater channels will invariably be rusted and this would accelerate rusting of the new base plate.