Re: wire guide
I - Goes to your key switch accessory terminal. This is the positive voltage source for the gauges so they turn on when you turn the key on. Purple is a common color for wires going to this terminal. In the controller you will find a purple wire running from this terminal on the key switch over to the warning horn. Often the horn terminal is a common point to tie gauge wires to.
G - Ground. typically black wires.
Light wires - One wire, the positive voltage wire, will go to your light switch and the other wire, probably black, will go to ground.
S - Sender each gauge has its own sender wire. You didn't mention what gauges you have. There are some standard color coding used for wires but these may not be the same at your dash due to custom wiring so you should be careful. Grey is common for your tach signal, tan for the temp warning sender (to the horn).
The lights' positive wires may all be hooked together (daisy-chained) in series rather than each individual wire going from each gauge to the light switch. the same is true for all you ground wires.
You said you have a purple wire, so you can assume that this means it is the 12V+ wire coming from the controller. That would be the 'I' wire for your gauges. I don't know about the other two wires. Are you sure one isn't grey?
Here is a link from th e iBoats FAQ section for a better explanation.
http://forums.iboats.com/showthread.php?t=186986
