teleflex volt gage wireing

[48tony]

I have a pego batery switch on my boat.how do i hook up my new gage so when i switch over batt 1 to batt 2 and vise versa i get volt readings. the volt gage has 3 terminals 1 ground .one has letter I thinnking ingnition and one has the letter s .and allso 1 plug for light.what is the best way to wire this teleflex volt gage. thank you tony