Antenna for ST 6001+ Autopilot

[mphem]

As I brief background, I have a Jeanneau 43DS, located in Kemah. A couple years ago, I installed a C140W and new radome. With help from the forum, I was able to interface all my ST60 instruments. About a year ago, I also installed an AIS, which works great with my C140W. I use the internal GPS antenna for my C140W and have another GPS antenna mounted on the stern railing, with the cable passing though the rail. I'm not sure where the cable ends, but I believe that it goes to my ST 6001+ Autopilot S3G Course Computer. My question is: the old antenna is looking pretty worn (been on boat, probably since 2003) and I just had made a new stainless steel antenna pole, where I have moved the AIS antenna, an iridium antenna and a KVH antenna. I would also like to install a new GPS antenna for the autopilot on the pole. Can someone tell me which antenna I would need to purchase and if I am correct that the rail mounted antenna is for the autopilot. Thanks for the help.
Mike