I am about to dive into my heater cores and find the leak. After reading a few other threads, I am thinking it could be the rear under the seat. That is where I am going to start at least. But I have a question about something in the engine compartment. What is supposed to go to the area I have pictured? When I got the truck, I was told by the owner that there was a small leak where you can see a small piece of hose with a screw clamp on a tee where the coolant goes through the firewall. I have never seen it leak, so I just left it alone. But now that I am going to possibly replace some stuff around there, I would like to know what that is, or was. Was it a valve? BTW, that hose is actually a cap closing off the tee.