For polarity checking, buy yourself a £9.99 multimeter from Maplin, set the switch to "volts - any number above 12". Make sure the red lead is i nthe + terminal on the meter, black to - (if the meter has built in leadsthey wil lbe the right way round already)
Now touch the probes against the two marked "ign" and "gnd", red to "Ign", black to "gnd". Your needle should go to somewhere around the 12V mark on the dial, or if it;s digital show a positive number n the scale. If the needle tries to jump off the scale or you get a -ve number, the supply is wired @rse about face.
Next job: Follow the wire connected to "send" & check every connector between tacho & engine. It should finish at a small black or grey box with three or four terminals, two of the other wires of which dissappear under your flywheel the other will probably be a red which makes its way to the big cable from the battery. The wire from the tacho should be on the same connection as one of the wires from under the flywheel.
Note that was an educated guess about how your engine is wired. If it's a nice shiny new computer controlled thing the chances are that will all be done in the engine management box, so shout back if it doesn't look like what I described.