I have removed all of the stock gauges and have replaced with Auto Meter gauges. The Auto Meter fuel level gauge is programmable but, I still like the low level light that was in the srock dash.
I believe the yellow wire from the top of the fuel tank is from the fuel level sender and is used by the stock dash to illuminate the low fuel level light in the stock dash.

If I hook one side of a 12V bulb to the yellow wire the other side to ground....will the light come on at low fuel level?

Tim,

Yellow/Black wire is for the warning light. You want to connect the following in series; 12V, a fuse, your light bulb, and then the yellow/black wire. When fuel gets low a switch closes in the fuel sender allowing current to flow through your light bulb to chassis ground.

-mark