1986 C10 heater core

I have a 1986 C10 that needs a heater core. As far as I can tell, I have everything ready to pull the box out, except for the one bolt on the
bottom right side. Looking under the hood, I can see a plastic(?) push in connector of some sort in the area.Truck has a/c so that is not very easy to get at. Any advice will be much appreciated. Thanks, Larry
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