instrument cluster ground
The factory engineers added the single ground at the headlight switch which is the same result as speedo. The problem is over time the screws loose their connection to the copper pad and to the pot metal threads so the total resistance accumulates and varries from one point to another. A tinned ring terminal against the copper pad is taking a more secure , direct, dependable path. Soldering the wire to the copper trace would be optimal only if vibration and the yank factor were eliminated.
Not so much electrical theory or proof from a ohm meter, just attempting to improve upon the next 45 years service while renewing.
A lot of ground wires that aren't routed in a proper manner might repeatedly break the plastic bulb sockets when yanked too so... Who knows whats the best plan.