If I can remember when I remove mine, I lay the dash across the steering column close to the opening and attach the ground wire, and then screw the Vehicle Speed Sensor (VSS). Partially place the dash into the opening and squeeze you hand inside to connect the harness cable and then the speed cable.
If you look the PDF file on page #5 you will see the back of the digital dash where the VSS needs to connected by the speed cable is also connect to. The ground cable is screw on the lower right of the dash. To make life easier, turn ignition on before tighten to make sure all is working properly