Could be a bad sending unit.
On each gauge, you should have 3 wires- IGN+, GRND-, and sending unit. I assume it's the sending unit wire you took off & grounded. The IGN+ and GROUND- wires usually have more than one wire, as they are run in parallel with the other gauges (all together).
When the key is on, the IGN+ should have power- check it with a dummy light (no offence). If it works, ground the dummy light to GROUND- if it works, your gauges are getting power and ground.
The first thing I would try would be to swap the sending wires on the gauges- if the problem switches sides, then it's the sending unit- if not, it's the gauge.
A sending unit in your tank is nothing more than a resistor and a float. As it floats higher, more resistance, the gauge reads full. Hence, when you grounded that wire fully, the gauge read full. Sending unit tend to fail because of the environment they're in- eventually they rust & die. I've replaced a couple in my time.
They make universal sending unit kits- you can get them at West Marine, etc. Check to see your bolt pattern, and go get a kit that fits. Read the instructions, as you have to adjust it for the depth of your tank. No lie- it's a 10-minute job.
Hope I didn't confuse you too much!
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