Can anyone tell me the main reason for tach failure? I figure its either cable with sensor end or tachometer itself. The rpm sensor cable attaches near the flywheel where it senses rpm.

The other end of cable looks like it plugs into some black colored box behind instrument pod in the dash. Some how there is something that would feed the pulses to the tachometer but I not see a place on back of pod where something like that plugs into it from the blk box. Can anyone help? Thanks.