What about the third wire on the switch? There should be three posts on a switch with a light built in. Two of them are the routing for the power and the third is an accessory (which means the light) which needs an extra 12 volt source/ground depending on how you wire the switch. My switch inside is only a 12v wire that is interrupted by the switch. The relay connects directly to a ground under my hood and then my interior switch runs from a 12v source through the switch and out to the relay. so my switch doesn't actually have a ground attached to it.