A battery at rest is 12.6V. If the battery is showing 12V it is just about dead and needs charging immediately.
I’ve had problems in the past where the rev counter did not read and it turned out to be a loose wire on the alternator. So it is worth checking that. However, if the battery really is at 12v and the rev counter is not working there may be something wrong with the alternator – a blown regulator may be? So worth checking the voltage at the battery (using a Voltmeter) with the engine ruunning – should be over 13V
Finally, I’ve had problems in the past with oxidisatiion of the wiring connectors on the control panel. This has required removing the panel, taking pictures of the wiring and cleaning up all the contacts, before putting it back together again using the photos as a guide.
I hope this helps.