Fuel and Temp Gauge Question??

I haven't seen one from the factory that didn't have a ground strap from the right head to the firewall. Even later models with added grounds from battery post to radiator support and elsewhere still have the first one installed. I have seen many that had that part left off with engine R&R.

Lets back up a bit too. If you have a low oil warning lamp instead of a gauge there, it doesn't get its power from the gauge "low voltage" supply.
The low voltage supply is only used for oil, water, fuel, gauges.
Other issues like weak charging system are not related.