Toggle switch Wiring... I am confused...

Three postion toggle switch one lead is for the signal (your lights), One is for the gound(chassis) and the other is for supply voltage( 12 volts). I think you want to use the accessory key position to turn on your under light bulbs with a switch. If so find the wire that is only hot in the accesory key position and use it as you supply voltage. This way the switch will only turn the lights on with the key in the accesory position and not when it's in run or off. I hope this was what you were getting at, hope this help.
 
A switch like that works like any other switch, run power to it and from it and the switch completes the circuit. The earth ground (just a regular ground) is just so the switch lights up when it is turned on.