1999 expedition 5.4 l. I've looked through my entire Haynes book and cannot find what it is. The hose appears to enter the cabin at back of compartment. Whatever it is seems to be leaking water, or at least a part under it

 

Leaking water or coolant? Any pics to post?

 

A pix is worth a thousand words. Not sure where you mean "back of compartment".

Back of cabin, glove box or firewall.

 

It's not letting me attach a pic so I can only describe it. It's behind and lower than the main radiator hose. It has a bar of some kind leading to the alternator 6 inches to the right. There's a hose off the top that leads to back of engine compartment, most likely part of heating or ac system. The hose appears to run into the area under dash. There's a silver ( I'm not sure what it is) long rod that looks folded in half. To the left

 

Above pic shows a heater core with 2 hoses. These 2 hoses will go thru the firewall from behind the glove box to your engine compartment.
There is heater core coupler joining these 2 hoses to your other hoses. Coupler shown below:



Do not buy the heater core yet, it rarely goes bad. You will need to identify where the leak is.
Not sure the type of coupler on your 99 expedition, on 99 E and F series, those are quick release coupler but pretty much one time use, everytime a heater core is removed for whatever reason that coupler should be replaced.

Can you at least identify and find out which hose (out of the two or both) is leaking?

 

It's not that, although it's possible for it to be something it connected to. I've tried posting pics but it tells me I can't post here. There is another hose that runs beside it into firewall

 

It's not a hose, but has a hose connected to top of it. It's leaking where it bolts to something else. Sorry I'm not experienced with some parts. I just can't find in my book

 

If you don't post pic nobody can help.
New members are not allow to attach pics, there is no such guideline here, perhaps your pics are too large in file size and it timed out when uploading. Look at this thread: https://www.fordforum.com/forum/ford...i-broke-49980/ our new member named NotTheEasyPath just joined and he posted a couple helpful pics already.

You said that's not a hose but with a hose connected to it, what material is the hose, metal or not. Do you think that component can be "heater control valve" or not?

 

It worked. I could post on other site. Now it allowed it here. The part with the arrow is leaking from bottom

 

3 location diagrams above hope you can locate the part.