Fuel Gauge Issues

Hi all

Hope someone could shed some light on this for me I have recent got an escort van w reg with 82k on the clock 1.8D 55 full service history been well maintained etc. The problem I'm having is when I got it the fuel gauge shown empty all the time since I have had the tank off and cleaned the wire etc with no luck I then followed the wire to find it not plugged in to anything. And found a white wire and brown wire handing. After going to a scrap yard every what had the white wire plugged into this so I've joined the wires together and the fuel gauge started working however it works in reverse shows empty when full and full when empty not a massive deal but annoying. I've tried swapping the wires round, earthing the brown etc and nothing works. Has anyone ever had this or know what I could check?

Cheers

Ian