got a pic of the sender
reverse reading could also be which way the float is set on the bottom of the sender and will change which way the arm will change resistance
so if you have the space in the tank try having the float arm coming out the opposite side it is now which will swap which way it will work.

but as stated only wiring will be sensor wire to insulated tab on sender (check no short between this and sender body) and then just make sure sender body is earthed. if sender body is not earthed properly grounding out the sensor wire won't do anything