^^^ x2

Now the $64M question - does the changeover valve energize (receive voltage) to send vacuum to the pod, or does it de-energize (no voltage) to send vacuum to the pod? It is important to know what the proper operation mode is.

Assuming the pod is supposed to receive voltage (energize) to send vacuum to the pod - If the changeover valve is energized, but the pod is not receiving vacuum, you probably have a problem with the changeover valve.

It could also be worth checking your vacuum source at the changeover valve itself. It's possible it could be weak, or you may have another pod elsewhere in the system with enough of a leak to reduce the vacuum strength to a level that the defrost pod no longer operates.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]