Autometer fuel guage not working

I replaced all gauges with Autometer and used the factory harness by modding the connector.

You've already verified that the gauge works by grounding the "S" terminal.

I would check continuity of the dark blue wire (fuel sender) from the front to the rear. Disconnect it from the sender and the gauge while checking. It should have little to no resistance. You should also find no voltage present in the wire while disconnected. If you do, there's a short circuit somewhere.

You also need to verify that the sender is working. (If it worked before with the old gauge, then it's probably fine). Make sure the ground strap on the sender is still in place and secure. If the sender loses it's ground, you'll get no gauge reading.

Just for grins, make sure you've got an adequate amount of fuel in the tank.

Once you do get the gauge working, bear in mind that the Autometer fuel gauges are very non-linear, unlike the stock one. It will behave differently.