Back up guides
 

Does anyone have info on how they work? one of mine refuses to go up. What is the system called?

Parking assist rods?

They're operated either by the closing assist pump or the central locking pump (which also runs the lumbar adjustment). My guess is the central locking pump which is a pressure and vacuum pump.

Sixto
95 S420
87 300SDL

Parking assist rods
 

where is the central locking pump located? Under the rear seat?

Back up Assist Rods
 

If one goes up and down I would check the other for binding. They are driven by the central, Lock/Unlock , Lumbar, etc pump under the back passenger seat in a foam cover. They are driven with pressure and returned by vacuum, orange tubing RH on the pump. be careful not to break off the fittings on the pump when pulling of hoses as I said before they are tied together with one hose.