Hello,
You should have a hot wire from fuse #5 going directly to the solenoid. It is energized anytime the ignition is on.
The pressure switches supply the "earth" ground (negative) to the solenoid whenever the transmission is in a drive gear or reverse.
The reason you only have one is, someone left one off ! Just hook up one for forward gears for now and run a positive line to the solenoid if you are missing it also.