Here's the write up taken from a review/how-to regarding installing Nighthawk LEDs in a C4. Unfortunately, I don't have the link to the original article.
The second part of the Nighthawk LED installation on a C4 is a modification to the fog light wiring behind the right-hand side of the dashboard. C4 fog lights are grounded through the high-beam headlamp filaments. When the fog light switch is on and the low-beams are selected, current flows through the fog lamp relay coil then, through the high-beam circuit to ground. That energizes the relay and the fog lights come on. When the high-beams are selected, voltage on both sides of the relay control circuit is equal, and current won’t flow. The relay is de-energized so, no current flows to the fog lamps, even if their switch is on. This system will not work when Nighthawk LEDs replace stock headlamps because the LED’s have no filaments.
The modification to the fog lamp wiring requires the light green wire coming out of the fog lamp relay be grounded directly to the chassis rather than through the high-beam headlamps. The relay is located behind the trim at the bottom right side of the dashboard in the relay retainer. You will need to remove the lower right-hand trim panel and the knee bolster which is behind it to gain access to the relay wiring. There are four, or, if it’s a ZR-1, five relays there and the fog lamp relay is front row, far right. Four wires will connect to it: yellow, light green, purple and brown. Find the light green wire and cut it and splice in a new section which grounds to the chassis. This will restore fog lamp function when the headlight are on low-beam and allow the fog lamps to be on when the high-beams are on as well as when the low-beams are on.
