gas gauge wierd hook up

[Razr]

Ok so I have the tan wire going from the fuel tank sending unit directly spliced into the tan wire on the harness behind the dash and the gauge works fine. However i disconeccted it from the harness behind the dash and connected it to the fuse box where it says fuel gauge and the dam gauge doesnt work it goes to full and stays there. Now another question wat is the cluster feed for because I have it on my fuse box but noting is connected to it.