IIRC it comes from the engine to a yellow check valve by the brake booster then into the cabin. I’m not sure if the same line supplies the folding rear headrests. Or maybe 126s don’t have a dash switch to lower rear headrests. There’s a vacuum switch bank in passenger footwell to control air distribution. It’s an involved process to check all the pods from there

Sixto
98 E320s sedan and wagon