Help needed on autotuner

[ad5it]

I have a autotuner that was given to me it looks like sea 1612c the webpage is as follows:
http://www.seacomcorp.com/pdfs/SEA%201612C_01-07.pdf
When I hooked it up I heard internal relays clicking so I know it was getting power, there is a question about the third line that has nothing attached to it on the power, ground, and third post if anyone has any idea what this does I would like to know if it should be hooked up either to a ground or dc.
I did not get the book and and cannot find anything on the tuner.
any help would be appreciated. I'm also looking for anyone who has used a sgc external auto tuner please contact me.