On the 51 it is wired from the light control switch along with the other instruments lights (36) on this diagram. Blue/Red wire on the left side of the diagram, 36e.
On the 50 if you don't have a clock I would use the clock illumination wire bullet socket. If you do have a clock, I would go to the bullet connector where the clock lighting wire connects in and use a multi-bullet connecter adapter and connect it there.