[9t9-5.2]

It looks like the connector for the low pressure switch that is usually mounted on the accumulator/dryer. Yet I see no pressure port...

 

[9t9-5.2]

 

[9t9-5.2]

I think i recall it being for the ambient air temperature sensor if you have the overheard display in the cab.

In my 99 that is behind the grill.