where to buy third or is it fourth light for rear diffuser?

 

more details please

 

Are you talking about a rear fog light?

 

ya maybe thats what its called bro.

 

Are you referring to what the Z has in the rear? I think there was a sedan on these forums that bought one from a dealership modded it to fit.

Is this it?

https://www.myg37.com/forums/showthread.php?p=3477881

 

yes that would be it.
where would it good to connect something like this to power it?

 

Tap into tail lights if you want to always be on OR tap into fogs if you want to come on with fogs OR put a toggle switch to manually power it on when you feel.

PS: The correct way is to run the wire from the light ALL THE WAY TO THE FUSE BOX however a ghetto approach would be to directly Tee off the tail light harness.