Have a 2000 W E38 750 with a rear foglight problem

Lights all work, front fogs etc, but when you use the left hand toggle switch on the binnicle it activates the front fogs but when you go to twist it that bit further for the rears they flash but dont stay on. So if you hold it they are on but other wise it just springs back without them working.

So bulbs etc are fine, switch works the fronts, any ideas on where to start?