Is there a drain in the heating system?
 

With my engine and transmission removed I recently pressure washed and degreased my engine bay. In the process I'm sure some water made it's way past the heat valve... where would the water go from here? Is there a drain in the system?

The heat from the exchangers passes through tubes inside the rockers, below the doors, and into flexible tubes that bend upward behind the side panels of the foot wells. If water entered as you said, it is likely pooling inside the tubes of the rockers--the lowest point. There is no drain.

If the water intrusion is significant, you might be able to snake a flexible hose into the rear opening (remove the air valves), and draw out the water with a wet/dry vacuum. If the amount is small, maybe just blow warm air through the system for several hours to evaporate what may be there.

You can hook up your shopvac to the heater intake on vacuum for an hour or two effectively. That shoudl do it

The ultimate way to do this, but only when the car is stripped, is to ducttape a leafblower to either side but you would NOT want to do that to a car with an intact interior.