Can you still drop your rearheadrest successfully when it is up by pressing the button on your dash? Please remember that this feature works only when the engine is running and there is a vacuum pump operating. The door locks operate whether the engine is running or not. Correct me if I am wrong. Total cost for this fix is a bit high for the labour required for dismantling all the parts and the dash to find the "leak". If you already know where the leak is, you would certainly reduce the time required to find it. The initial search is to start from scratch i.e. could be anywhere in the whole system of vacuum lines around the car. However, it pays to start first from the vacuum units, then those diaphrams or whtever you call those things, then look at the hoses around the car. When you say "to no avail" I would assume that you could not move the rearheadrest down at all?