Eric,

I am traveling and can't look at my stashed-away rear heater for a procedure. But I can tell you that one of the problems you're going to have is keeping the rear seat in a position where you can work on the heater. This is important because not taking precautions could break your arm or maybe worse. You've got to hold the seat up with a 2 x 4 or similar, or else you need to remove the seat from the car. There may be some offers of shortcuts out there but it's pretty difficult to get the whole thing out with the seat in place. You can do it, but you have to hold the seat just right. 

Jim