Need help understanding rear head rest solenoid
 

I have a 1991 300E (W124) 3.0. It has a solenoid under the drivers side rear seat to switch the vacuum from the engine air pump line across to the lines for the rear headrests to release them. This switch has 3 male ports one for the main suction and one to each headrest.They are vacuum actuated. All the switch does is open and close the valve. I have suction at the main line. If I connect them directly the head rests drop. I have 12V on two of the 3 leads from the 3 lead plug without pushing the dash button. If I push the dash button the 12v jumps to the third lead. The switch works on the car from which it came from. What am I missing? Mine does not work. Can someone explain what I can do to trouble shoot further? Thanks...JRR...jrr4948@optonline.net