Heater working on/off in '87 560SL

[2quik6]

The heater has started working intermitently on the '87 560SL. I was told it may just be the diaphram and takes only a few screws to replace. Does anyone know exactly which part this may be and where its located?
The shop that did alot of work on this car earlier in the year told me it was a simple fix if I bought a rebuild kit of some sort and it should not take longer than a few minutes to swap out only the diaphram part or they could do it. I'd prefer to do it myself if I can figure out which part to get.

Thanks