Re: Fuel and idiot lights, here's how!
OK, I had a PM requesting a bit more info, so I thought I might as well provide it here where it's available as wisdom for the ages.

I had hoped that I could look up all of the necessary parts on Digi-Key, but I'm not enough of an expert to be sure that the parts I look up there are actually correct, so instead I'll just share what I did and maybe someone with more specific knowledge can step up and provide some links to parts.
Also, this stuff is cheap - if there are folks out there who want the same setup, I'd be happy to put together a little kit of pieces from my local electronics shop. The kit would include:
1) Ultra-bright 3V 20mA white LED.
2) A holder for the LED.
3) 47 ohm 5W ceramic power resistor (the load resistor).
4) 470 ohm metal film resistor (the current-limiting resistor).
Cost would be somewhere in the range of $5 as I recall the prices of those things. Plus shipping, of course. I don't intend to put together a bunch of these and pricing would not include any profit for me - just happy to help out a few others who want to take this same approach for whatever reason.
If you want to pick the parts on your own, it works as follows:
1) Use that same 47 ohm 5W load resistor in any case.
2) Pick your LED. It will have 2 properties that matter to us, the forward voltage, and the current. In my case, it's 3V, 20mA.
3) Calculate the current-limiting resistor's value as follows:
3.1) Voltage drop = 12V - LED voltage = 12V - 3V = 9V.
3.2) Current = 0.02A
3.3) By ohm's law, 9V / 0.02A = 450 ohms. Round up to the nearest available value = 470 ohms.
3.5) Power = 9V x 0.02A = 0.18W. Round up to the nearest available value, I think mine are 0.5W.
4) Wire it according to the diagram I showed above - the current limiting resistor goes in series with the LED, while those two together go in parallel with the load resistor. That whole assembly gets wired in series with the thermistor you want driving the LED (either the high-level "get gas soon" or low-level "get gas NOW!" sensor).
I'm trying to get my whole dash together this weekend, I'll update here soon with pics of my test rig and the progress of the finished product.