Frustration ! In cab fuel sending unit inop.

[54blackhornet]

I recently bought the correct send unit for my 71 C/20. Following the instructions I inserted it along with new gasket and sealing ring. Tank is grounded well to cab. Tan wire from sending unit is plugged into the fuel gauge input on fuse panel. With key on gauge is stuck at 3PM. Disconnect and grounding tan wire gauge still stuck at 3PM. I even tried grounding the sending unit directly to the negative battery post, nothing ! Would pulling the firewall connector where all the wires enter cab help ? Loose connection or corrosion ?