3 post illuminated rocker

I am installing a 3 post Illuminated Rocker switch on a boat and the third post is confusing me. The switch controls a relay to turn on and off the trolling motor power. I can get the relay to function properly via the switch but can not figure out the configuration to also have the light in the rocker switch illuminate when switched on. My set up is 2 12volt batteries run in a series. Thanks in advance for any info.

Per the request of the system this post is regarding to a electric power prototype boat so I dont believe year make model will be relevant.

Are there any markings on the 3 terminals?  Are you switching power or ground?  Do you know if the lighted switch is an LED or a standard light bulb?

If switching power, apply ground to the 3rd terminal.  If it illuminates all the time, reverse the other 2 wires.

If switching ground, apply power to the 3rd terminal.  If it illuminates all the time, reverse the other 2 wires.

Yes, from left to right they are +, A, and a symbol that looks like half a plus sign pointing up and 2 small lines beneath it. I believe I am trying to switch power. The relay has 4 posts total. 2 small and 2 large. Here is a link to the switch. http://www.amazon.com/SPST-Automotive-Round-Rocker-Switch/dp/B001TQKKMC thanks

The light is LED

Plus = 12 volts in.   A = Accessory or 12 volts out to your device.  That symbol with the 2 dashes at the end of the line is the universal symbol for Ground.  Notice that the end dash is shorter than the previous one, it is an arrow pointing down, to the ground.

awesome! Makes sense thank you. Seeing as I have almost no metal on board, would the negative terminal of the battery in the 24 volt series work well for the ground. I believe it would prefer not make guesses.

Ok, ran into a couple problems with this approach (because of my mistakes). + = 12 volts in from positive side of battery. Done A = Accessory line to one of the 2 smaller posts on the relay. Done Ground = Ground line run to negative terminal of the 24 volt battery. Done. The other small post of the relay I have running directly to the positive terminal on the battery. The 2 problems I am facing are: 1) light in switch is constantly on. 2) relay does not function. I have the 2 big posts on the relay intersecting the main + line just after the circuit breaker. Thank you

that did the trick. Thank you for your assistance!