[Juergen]

If you ruined two relays (and they were grounded properly) it would seem they carried too much current. Now too small a wire increases resistance so they should experience less current. Too small a wire decreases the voltage at the load so the fog lights would be dimmer. Now if you had a short in the wiring or lamps, the current would increase drastically and burn out the relays. I'd check the wiring and run the lights at the battery to check the load.