Pulsating Interior Lights and Voltage Gauge With Turn Signals

We are working on completing a full restoration and when I turn on the turn signals, the interior lights and the voltage gauge pulsate. In addition, they dim when pressing the brakes. I have tried it with the car running as well and the same results. Here is what I have tried so far:

1. New Battery was installed recently and is fully charged
2. New Alternator was installed recently
3. New headlight switch
4. New printed circuit board on back of instrument cluster
5. Scraped off the paint to make sure my grounds have good connections
6. Unhooked the hardness for the taillights, turned on hazards and same results which leads me to believe it’s not in the rear lights.
7. Verified the following grounds:
   1. Right and Lift side headlight grounds coming out of harness to core support
   2. Ground from battery to the front of the block
   3. Ground from the back of the engine to the firewall
   4. Ground from the transmission to the frame
   5. Ground connected to the dash from the instrument cluster
   6. Ground from the rear lighting harness

Is there somewhere that I could hook up an additional ground to verify if it is a grounding issue or to solve the issue?

Thanks for the suggestions as this one is really starting to frustrate me. I don't want to put the rest of the stuff in the dash and interior until this is resolved.