To follow up on this in case someone else needs to know, I went ahead and tracked the other side of this to a gland/bulkhead port behind the driver's footwell trim under the center console. That line goes to the cabin air temp sensor. The same type of bulkhead connector is on the engine side.

It does just pull off, but my line decayed to the point of leaving some in the recess between the outer and inner port.

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2001 E53 3.0 5pd