Defrost?

[CREATURE850]

I have 04 4.6 V8, if I use the defrost it works fine but when I shut the truck off there is a hissing sound on passenger side dash. I also lose water on a normal basis but more when using heat. Any ideas? I am also looking for a diagram for a train horn install if anyone has one.

 

[GRKN]

Leaking heater core. Pull the dash and replace it, not too bad a job really, I’m sure you can find videos on YouTube.