Troubleshooting climate control for 1979 300D

[akreis]

I am trying to troubleshoot the climate control system in my 1979 300D. The car has 188K miles on it and has worked flawlessly until it stopped. I am suspecting the climate control amplifier but would like to test components of the system prior to just purchasing new parts and installing them. Does anyone have a test approach that would help isolate the problems I am experiencing? Thanks in advance.