Challenger 1800 Reverse Cable Nut

[Cronullalad]

Hi All,
The Shift / Reverse Cable on my 1998 Challenger 1800 has seized so I am going to replace it....my question is how to remove the Cables Locking Nut at the back of the boat. Its a really weird shape so I'm thinking it must require a special tool that locks into the webs around the circumference of the nut.
Any ideas on how to remove the nut would be greatly appreciated.
Thanks.

 

[rusty nail]

the nut is on the inside of the bilge. no need to turn the piece on the outside.