Veggie tank fuel gauge issue

I'm having problems with my Greasecar fuel gauge. I acquired the car (a 1985 BMW 524td) recently and the seller was the one who installed the kit. He said the gauge stopped working a few months ago. The fuel gauge needle stays about 2 or 3 millimeters above the "Full" notch. It moves back and forth very subtly and fluidly, but only about a millimeter left and right. Where should I start looking with regard to troubleshooting or replacing parts?