Tough Question: 94 Cav, 2.2L Cowl Hood Heater Fan Unit

[Tower1980]

I didn't know what to call it, so I'll describe what I've done.
I've removed the cowl panel (between hood and windshield). Underneath, I've removed the black plastic shell cover on the passenger side. Underneath that, to clean out the twigs/leaves/garbage, I've unscrewed a unit that has an approximate 3" by 6" hinged flap door which opens to the heater fan. Underneath this unit with the flap door is an open hole leading to some wires and further to the passenger side floor panel.

My question follows: when I raised this flap unit from it's hole, I noticed two tubes - one black and thin unconnected, and the other orange and thicker (approximately 1/4" in diameter), connected to the bottom of the flap unit; the orange tube I predict to be a vacuum line for this flap unit. The thin black tube was disconnected, leaving me worried. Can anyone tell me what this black tube is for, and if it's ok left unplugged from wherever it came from?