[Accelo]

These trucks tend to plug the water side of the heater core. I am not sure why it happens but it does. Just helped my friend do his, night and day difference in the cab heat.
Just pull the heater hoses and back flush with a hose. No high pressured needed.
Hook it back up and enjoy. Be careful as the plastic quick disconnects get fragile with age. It takes a special tool to get them off.
Stainless steel replacement are available but not necessary. Just be aware of the possibility of them braking one off. I recommend replacing them to avoid issues later.
I had to do mine about once a year till three years ago I flushed the system and replaced the antifreeze. Been good since then.