73 Duster after market fuel gauge with stock sending unit

So, I did some testing this weekend and this is what I found.

When I hooked up a grounded test light to the wire that connects to the sending unit. The light on the sending unit blinks at a rate of about once per second. When it is grounded the fuel gauge would slowly rise, and stop at about 1/4 tank. I hooked the sending unit wire back up to the sending unit, and added an additional ground from the sending unit to the frame, which didnt help.

The sending unit is less than a year old, so I would think that is not it...Any ideas what the issue is?