So I've identified the wire that goes to the gauge cluster connector behind the dash, the White/Green wire.

I used my multimeter to verify its Ohms and it looks correct. The tank is almost empty right now so the wire is reading about 62ohms.

However, once I turn the Ignition to ON and the fuel pump turns on, I'm not getting any resistance reading from my multimeter on this wire anymore... what's the case?

 

You shouldn't be able to check resistance with the power on, only when there is no power going to the wire. Is your gauge not reading correctly?

 

I'm in the process of wiring in an aftermarket dash, so I'm hooking it up to the stock fuel sender's wire.

How will my aftermarket dash read the resistance values then from the fuel sender when powered on and the car is running?

 

It should read the amperage going to the wire, the resistance will affect the amperage either raising it if you have less resistance or lowering it if you have more resistance. Those to values will tell the gauge where it should be at. If you are reading correct resistance at the wires when the power is off then you should be fine as long as the gauge is compatible

 

Take dash out and ground the rpm gauge screw ( labled gnd )
there are 3 screws holding the rpm gauge in place one is clearly labled gnd
My gauge had a earth fault my spedo never worked also
This sorted myn
Actuly done it last week lol
Good luck hope this helps

Ive played with a cheapy after market fuel gauge it worked but i had to use the sender wire and use the ground on the fuel hanger to gauge