Erratic tach

Customer sent me this video showing his tic toc tach jumping all over, looks like the brake warning lamp is going bright and dim also. Any idea what to look into?
file:///var/mobile/Library/SMS/Attachments/86/06/F5702CA7-94FF-4A09-BD77-865E55B219BB/20250806_140926.mp4

My knee jerk reaction would be to look at dash grounding

HawkRod said:

My knee jerk reaction would be to look at dash grounding

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Is the tach grounded separately or does it share a ground with the clock? Or just through mounting?

dan juhasz said:

Is the tach grounded separately or does it share a ground with the clock? Or just through mounting?

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It's been awhile, but I think it is grounded through mounting.

Saw the customers 69 charger at a car show today. He showed me what is going on. If he turns on the blinkers the tach will drop with every flash of the blinker. Couldn’t do anything at the show but does that sound like it might be a ground issue? I looked under my 69 sup bee dash with tic tok tach and someone fabricated a simple ground wire from instrument housing to chassis .

I noticed the amp meter never gave a positive reading. It only showed if there was a draw . I checked grounds at the aftermarket regulator and checked the power feeds that passes through the firewall junction. The fusible link terminal in the connector was starting to get hot and started to damage the connector and it was slightly backing out from the damage to connector. I cut off what was left of the terminal crimped a new one and also replaced the regulator with one of the blue mopar race voltage regulators. It immediately showed a charging amp meter upon start up. Most all of the problems I noticed with the blinkers being on diminished but did not entirely disappear. The tach used to surge ( needle bounce) with every pulse of the blinker, the courtesy lights and the e brake warning light would also dim and brighten along with the blinker operation. After the changes I see on the first blink the tach bounces a small amount then its stable after the initial blinker cycle. Do you think I still have a high resistance with the fusible link connection on the under dash harness side, could there be a flasher issue, maybe a issue with the amp meter itself. Looking for ideas on how to really get to the root cause of this problem.