They are spread around the evaporator/heater case.
Behind the glove box is the row of switchover valves that control them. You will need a good vacuum schematic. Just disconnect the line of proper color for each unit to be tested and apply vacuum to it. If it holds the element is probably good. The linkage occasionally become disconnected so running the blower and watching the changing function will finish verification.
Bosch Master, ASE Master, L1
33 years MB technician