The relay is powered up and activated thanks to a small electrical current. When it is activated, it lets a current that is much bigger than the original one flow through other two outlets. It is used to control the headlights, the rear fog light, the electric fan, etc.
The AC has dropped out and everything suggest it might be the MG CLT relay that needs replacing but I can’t find it - it’s not under the hood. Where else could it be?