First of all, the gearbox that you now have on the car, was it originally working with electronic sensor or hard square cable ?
If it was electronic, your electronic sensor that comes out of the gearbox should have 3 pins !
Test that with a multimeter. You should have 1 ground cable, 1 power with ignition on and 1 signal.
If you have both ground and power, a good way to test the signal is if you put the car on stands - be sure that its safe, and connect a rev meter on that signal cable.
Put it in gear and see if the rev meter works.
If it doesnt then you have a problem with your sensor.
If it does then check your connectors, spray some contact cleaner on the pins and try again.
You may have a bad connection at some point.
I cant remember the cable colours right now but I have the wiring diagram of the 33, should be relatively easy to find them though anyway.