This may be a little off topic, but I remember hearing on Cartalk a caller whose dog barfed down his defroster ducts. Click and Clack told him to sell the truck.
If you run the defroster for a while on hot, everything should dry out. How much did you spill down there?

 

I doubt there's a drain as such. Most defroster setups I've seen use hoses that look like miniature dryer ducts to get from the heater box up to the defroster vents. You might be able to pull them off from one end or the other, but this is speculation on my part. If it was mine, I'd set the controls to full hot blowing out the defroster with the blower on high and let the thing idle for a half hour. That would be enough to dry up a gallon of soda.

 

I would not worry too much about it. The liquid should run out the AC condensite drain which is behind the left front tire area. When I was running with a bad heater core a lot of coolant was entrained in the system, had to clean the windshield daily. Smell went away a few days after disconnecting the core. Air conditioning creats moisture and that moisture has to get out somehow. This is designed into the system.