Is there an OEM style heater box for our trucks?

[oldsmobilekindofguy]

Is there a OEM style heater box for our pickups that goes under the dash, It might not be called a heater box but the heating duct that goes to the defroster and vents.
If so, where do i get one?

Also if I get a new heater core does it come with the plastic cover.
Could you believe a mouse or rat chewed completely through the side of the plastic cover and through the fan blades of the blower motor and made a nest in the box, it was about a 2 inch hole. That is why I have to replace it all because it ws mouse infected.