There's a bunch of threads on the rear heater delete or getting it to work. I think you need to search and read some of them and compare pix to what you have or don't have. I have no idea what's been removed or plugged in your system.
Pictures above are what a stock system should look like. If you're missing stuff, post wanted ad in the classifieds.
There are pipes underneath the truck that run to the rear heater. Are those there? (they're usually removed).
You're going to need to buy a silver star subscription and post more pix otherwise you're limited to 10, I think. It's all just guessing without good pictures.