I need help please.....The rear headrests on my E220 lock upright ok but as soon as the engine is started they both return to the stowage position. I have checked the "stow" s/w on the facia and that is just fine. What I need is some info on the system....I understand it is a vacuum operated system but is there a valve in that system that could be "frozen" in the stow position..? The Haynes manual shows nothing can some kind person advise please, thankyou