I have a 1985 Mercedes 280 sl. The center arm rest (betweeen the drivers and passengers front seat ) will only stay in the down position. When you go to flip it back to get it out of the way it won`t stay up . There doesn`t seem to be a locking mechanism.Actually it looks like there may be a part missing. Any ideas on what holds it in the up out of the way position. Thanks for any help in advance. Regards Bill