You need at least three pins: one for power, one for ground and one for the communication to the unit.
I think at this point you will need a new GPS Receiver but let’s be sure by testing your 998 unit on someone else’s boat that is also running a Humminbird with an external GPS Receiver. It does not have to be another 998 unit as any of the GR series of Humminbird GPS Receivers will work for this. You can even test it while the boat is on the trailer. Just be sure to plug up the power, transducer and GPS Receiver even if you have to do so by hand.