I bought this gauge for only $20 because the seller said it was incomplete. But, he could not tell me what was missing. It appears to just be a wire harness that plugs into the back of the gauge. Can anybody here tell me if that is all it is? Here are pictures of what I have:

 

missin the wiring harness from the sending unit to the guage

 

but that looks like a pressure SWICTH not a pressure TRANSDUCER. If the pressure thingy only has two wires its a simple switch and has nothing to do with the gauge. If it has the same plug as the back of the gauge then its a tranducer and youll just need the harness. If its just a switch...Youll need a transducer. i think they are around $75 from various places.
Then youll need the harness to go with it.
That other thing that looks like a switch would be for an NX autmatic bottle heater.

 

in short you need the sending unit too