Take a look at this post and the article that bullit99 has. Read it carefully.
http://www.cuda-challenger.com/cc/index.php?topic=23985.0DO NOT hook up 12v to the fuel gauge!
I read it quickly and it looks like 12v goes to the voltage limiter which in turn limits the voltage to 5v.
The Fuel tank sender acts like a "valve" that increases and decreases the continuity to ground by resistance. Makes sure you Mopar sender has resistance readings 61-85 ohms empty and 8.6-10.6 full.
I would hook up everything then while having the sender out of the tank (once it's tested for resistance) and move the sender arm up and down and see if the needle on the gauge moves correctly. This will acurately test the gauge.
Also make sure your ground is good and the wiring is good also with no breaks. Mine had some mouse nibbles taken out of it.
Not sure if this is the best way but just what I thought of quickly.
Hope this was helpful!