67-72 Chevy Truck fuel gauge diagnostics 101

[hotwls13]

Trying to finish a blazer tank swap on a truck I just bought. They left the tank swap about halfway done. My sending unit has the ground wire coming off it (I attached the other end of that to a tank bolt). I don’t have a wire connected to the center post/bolt on the sending unit. What wire should I be looking for? Tan wire that would have ran to the tank behind seat??

I haven’t looked yet, hopefully it’s just back there and I can add some wire to it to extend to the back.