W126 vent question and possible vacuum leak

[Homeline]

Love tinkering with my 420SEL. Just this week I noticed that the rear seat vents only blow cool air, even when the front vents blow warm.

From what I can tell by the owners manual, the front air outlets all seem to operate as they should with respect to which vents (dash, center, side, foot well) should be blowing depending on what individual button is depressed. All vacuum systems (sans this one) seem to be operational.

Is there a separate vacuum pod for the rear vent? Loose control cable? Curious if the rear vent can be not operating properly while everything else is? I did have to get into the center consol recently to replace some burned out bulbs.

Could this be related to a vacuum leak type sound eminating from somewhere behind the headlight switch (US spec/no tilting lights) that is only audible when idling in park and dissappears the second car is put in any gear?

Cliff