I just swapped in a new bulb into my 3rd brake light and it now works. Since I had a 2 pack of bulbs I put the other 920 into one of the white bulbs on either side of the 3rd brake light. I can't get these white lights to turn on, I'm not really sure how. I think its a cargo light? What are they even used for and how do they get activated?

 

Yes, they are cargo lights

They should come on with dome light when you turn Dimmer wheel all the way "up"
Its to light up the bed at night

 

hmmm wow I just went and tried that. So it comes on with the cab light switch but not the door switch? Must have a separate circuit controlling it then. I'm curious why it doesn't come on when the doors are opened

 

Yes, it uses separate pathway

If 1995 - 2003 the GEM module activates interior light relay when door opens, and that turns on the dome light

The dimmer switch powers the dome light and cargo light directly, no relay

 

Also that way for less power draw. You wont always need the bed light on when the doors are open, and would otherwise be required to keep the doors open if you wanted to use the bed light