The black wires are usually daisy chained grounds for lighting.
Voltmeter Gauge wiring here.
https://www.vdo-instruments.com/medi...0Voltmeter.pdf
You can add a ground to the cowl square bar behind the dash with a #21 x 12" bit.
https://www.homedepot.com/p/Drill-Am...1221/305287325
Tap to a 10-32NF Or use self-tapping screws (not a good idea).
If going to 10-24NC use a #25 x 12" bit
https://www.homedepot.com/p/Drill-Am...1225/305287319
I like to use tap sockets
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1