I recently bought a new switch for my foglights. I already had a switch for them, but this new one has a LED in it. The old switch only required two wires to go to it, the 12V and Ground.
I get this new switch, and it requires three!? They are labeled:
1. to ground(obvious one)
2. 'to source(max 15A or to relay's trigger (+)'
3. 'to power(+)(12V)

I tried the hot wire to both '2' and '3' but the fogs won't light up...just the LED.

My question is, do I need a third wire, and if so, where should I run it from? Thanks in advance!!

 

the ground wire is just a ground for the light in the switch

 

3 will go the battery (power source), 2 will go the the relay's coil. Those two are what you need to switch the relay the same way as your old switch. The ground is to make the LED light up.
This is all assuming that you have the relay hooked up a certain way. For example, you have one side of the relay coil directly tied to ground, and you're using the switch to connect power to the other side of the coil.
The other common way to wire a relay is to put power directly to one side of the coil, and use the switch to connect the other side to ground.
Basically, you can either switch the power or switch the ground, so depending on which way you have it, that switch may not work right away.

It's easy enough to make it work though, just rewire the relay to use the first method (one side of coil to ground, one side to the switch) if it isn't like that already. Relays are really simple, so if it doesn't work, it's something really easy to fix.