1969 C10 cargo lamp

[bvb69C10]

I recently bought a 1969 C10 pickup. My question is this, how do I turn on the cargo light on the back of cab that shines into the bed. There's a cargo light on off switch in the cab but that is either broken or there's something else I need to do like turn the light switch on the dash until the cab light comes on as well to make the cargo light come on. Any help would be great to tell me how it's supposed to work.
Thanks