My E220 1995 coupe 2 door just failed its MOT due to the front seat backs not being able to be fixed in the upright position. They tip forwards but not backwards. There is a button on the upper seat back which is supposed to release the lock when you want to let rear passengers enter the back seat. When the seat is put upright, I assume it should lock, but it won't. Any idea on how to check if this electrical or vaccum problem? How to I trace the fault?