It's not behind the dashboard.
It's by the driver's footrest, behind a plastic panel, it's usually tucked away, so you'll have to rummage around there.
If the previous owner removed it or something for some odd reason, look at the wiring diagram in the factory service manual.
http://www.nicoclub.com/FSM/240sx/91-94/
Download all the PDF files in there into a folder, FSM is filled with great stuff.