I need to replace my defroster chamber but i cant figure out how to get it out. Do you have to take out the heater box which, by the way leaks, but thats besides the point.
Thanks,
1966stang

 

the plenum is held on by 4 screws, you can remove it without moving the heater box, BUT it is easier to loosen the heater box...sounds like you have a stock cardboard one, rip it out, the top 2 screws and the front lower one is easy to reach. I have swapped a few plenums and put only 3 screws back in to save time

 

You can also take out the glove box and the top speaker to gain better access to the hoses which are probably toast.

 

If the heater box is leaking you should just pull the whole thing and replace the heater core at the same time.