AC or heater core?

[pepperref]

I had water (or something else odor-free) drip on my right foot while driving last summer. The AC was on at the time. I later read about the heater core leaking coolant into the driver side footwell and figured that this was going to be a real PITA to replace.

I hadn't done anything about it and then last week a mechanic showed me something strange while he had my 190e car up on a lift. He pointed out the AC drain and told me that he had never seen one with a rubber hose wedged into it. Picture a piece of vacuum hose wedged sideways into a flattened plastic drain.

Thinking back to last summer, I realized that the water wasn't hot coolant, but COLD (AC?) water!

Is it possible for a plugged AC drain to overflow back into the cabin? If not, what could explain the cold water?