Well a reverse light isn't supposed to be fitted above 1200mm on a vehicle so i assumed that its NOT wired into the reverse light circuit. And if that's a work light, then its not to be wired into the reverse light circuit, and from memory (engineers requirements for my old rangie, and mr plods in a mates disco one day) the work light switch can't be accessible from the drivers seating position. So why would you put it on a relay?

