Thank you, both. Those links do help. It's the heater blend door actuator I am looking for. This link has the plastic connector
https://www.classicparts.com/1983-85...ctinfo/68-248/
There's a billet aluminum option at the other link from Getter-Done.
I'll keep reading and removing parts.
I'm not sure why my heater core would be blocked. I have my suspicions since there was stop leak in the coolant when I bought the truck. I thought it was for the radiator which did have a leak. Fun times figuring all this out.
Thank you.