The wiring for the cargo light is accessible by removing the driver's sill plate and looking in the wiring retainer underneath. I believe orange supplies power and white is ground for the light. The white wire is hot until it is grounded by the dome light turning on. All you would have to do is tap into the white wire coming from the brake light switch (hot when depressed), then connect it to the orange wire and ground the white wire coming from the cargo light. As for handling the leftover wires, simply unplug the cargo light plug in from the convenience center and you should be good to go.
Convenience Center diagram:
Fellas! I've got a '89 K1500 regular cab that I'd like to swap rear defrost onto. Truck currently has the dumb two piece slider window and of course the tab to keep them closed is busted. I would rather have a single piece window and the idea of defrost is groovy but I've only seen it on '95+...
www.gmt400.com
(Side note: I installed a later model 3rd brake light on a pre-94 truck, and discovered that the dome light plug-in will operate normally when plugged into the cargo light location, but not the other way around)
As for the bulbs, I have a white one of these in my pre-'94 cargo light right now and it works fine, red ones are available:
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