Probably #23 in the diagram below, 15971641 is the part number, seems like it is discontinued.
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Diagram of OEM HOSES & PIPES/HEATER PART 1-FRONT. Contains 8 parts priced between $9.57 and $52.94. Fits 1996 Chevrolet Tahoe Base 4DR.
www.wholesalegmpartsonline.com
Here is a similar thread.
Was in the process of replacing my fuel pressure regulator and I snapped the small plastic line that runs from firewall on the passenger side to what I believe is another sensor of some sort over by the TPS. I think is vacuum related Im not entirely sure. I just know its old and as I was trying...
www.gmt400.com
Sounds like others are have used vacuum tube/hose to replace it.
Looks like it is for controlling water flow to the heater core.
Best of luck.