You said before, tacho backlight works with both +ve and -ve terminals
But my conclusion is , this Tachometer's backlight wont work with the +ve terminal.(You will understand when you try connecting the tacho backlight wire directly to the +ve terminal of the battery, it wont glow. But it will if you connect it to the -ve terminal of the battery.):-Correct me If you get it wrong.
According to the circled picture, the connection you have made is to the wire coming from the 'parklight switch.' Here this point will get +12v only when the parkswitch is turned ON.
When the park switch is OFF, there wont be any voltage in the other end of switch. So here the rest of the circuit (I mean circuit after the switch will act like earth, since this wire has connection to the -ve terminal of the battery via all those backlight bulbs of your car) will help the tacho signal wire to make the circuit complete. So the tacho backlight will glow.
When the park switch is turned ON, tacho backlight wire will get positive volt via switch and it will make the tacho not to glow.
You will understand well when you try connecting the tachobacklight wire directly to the positive terminal of the battery as I said before.