Do---noo. You should be able to eliminate some of it. The ground should have continuity at zero ohms to some part of the circuit board--part of it that "looks" like ground, IE traces that occupy a lot of area. or even the case
You should easily be able to trace out the light, especially if it's removable. If it's wired either like my post, or the way the link you posted, ground should be one side of the light.
So you should be able to "try" what you think is the lamp, remove it, and the continuity should go away.
Once you have that, NONE of the other wires should show continuity of any sort to the one wire to the removed lamp, and the ground side should show "something" depending on which way your probes are placed.
In other words, let's say you suspect white to ground is the lamp.
Hook your meter up, and if you get a fairly low reading, remove the lamp.
So is white ground or black?
Hook your probe to white, with the lamp removed, and try the other leads. IF white was the "hot" lamp lead (lamp removed) there will be no continuity anywhere, IE "infinity" resistance.
IF black was indeed ground for the lamp as well as the tach, then the other leads should show some resistance, maybe high. Turn your probes around (reverse polarity) when checking the other leads to your "suspect" ground.
I think it's a pretty sure bet that red is power.
You might ohm out what is now left over, and see what you find.
