407 Rev-counter. Stopped working

[David C]

There are two options for attachment of the loop fed from ignition switch to the ballast resistor onto the tacho. Either the white wire loops around a plastic block which can be unscrewed from the back of the instrument or there are two bullet connectors and the loop is inside the tacho.
If the feed had failed I don't think you would have any sparks, so once you have checked Roger's suggestion of a bad earth it looks as though the fault is internal to the instrument.

Are you using an electronic ignition? I fitted a fancy uprated Mopar ignition which doesn't work with the original tacho. This requires conversion from current to voltage sensing. A DIY kit is available from Spiyda Ltd. Dependent on what is wrong with your unit this may be a solution for you.