Trunk light only illuminates when the interior lights are on (lid open obv).
Wiring diagram will make it faster and easier to track down the switch and rewire for trunk open/closed functionality.
Any help would be greatly appreciated.
You can get one here:
IIRC, the trunk light switch is what the bulb plugs into.
Tester
This might just be a feature so that a faulty trunk switch can’t provide a parasitic battery drain. You can always identify a circuit that is always hot and spice into it. Although one does not come to mind off the cuff.
Now to redirect, I have found trunk lights to be pretty worthless anyway in the dark. Menards sells a pack of four led lights with what looks like a wall switch. They are very bright, have magnetic and Velcro backing and replaceable batteries. I have put two of them in each car and they work great. $5-10, don’t remember. Put one by my power panel too. Stick right on trunk liner material or metal. Problem solved.
I’ve already replaced the bulb with an LED that’s plenty bright 
Since the trunk courtesy light works when interior lights are on, seems unlikely the trunk-lid switch is the problem. I’m guessing this is a body control module problem; either that or someone has inadvertently changed the car’s lighting controls. Aren’t there Chilton’s and Haynes repair manuals for your car?
There isn’t a “problem” at all. It’s functioning as it was designed, as far as I know.
I simply want to rewire it to work independently.
I’ll tear into it when the weather gets warm. I was just hoping to look at a wiring diagram for now.
Not in this case.
Ok, so as I understand it, you want the trunk light to turn on whenever trunk is opened, irrespective of passenger compartment controls. hmmm … One problem you may run into, there may be no 12 volt “on all the time” power cable routed to the trunk area.
If you can solve that problem (by running the wire yourself for example, fused circuirt of course) sometimes a mercury switch is used for trunk lid open/close. If so you might be able to see the switch on the underside of the trunk, small cylinder-shaped gadget.
This is where that wiring diagram will shine
I understand you’d like a wiring diagram, but I have no such diagram. And unless you can find it using Google, unlikely anybody here would be able to post one, b/c that sort of info is usually copyright protected. I was able to find a used Chilton’s repair manual for your car for less than $6 via Google search.
Coincidentally I tried to do this very same thing on my Corolla one time, ran into the no 12 volt power cable issue, and decided to put a flashlight in the trunk … lol …
Not the right one.
This is, just incase someone else is looking:
https://www.thriftbooks.com/w/general-motors-buick-regal-chevrolet-luminaolds-cutlass-supremepontiac-grand-prix-1988-2007/363642/item/#edition=5996536&idiq=3929350
Don’t trust a Haynes or a Chiltons for wiring diagrams.
I learned that a long time ago.
They’re not accurate.
Tester
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I can’t think of anything in the trunk area of your vehicle that would have B+, all that is back there are brake, turn, stop, running and reverse lights if the trunk light is on the dome light switch you are just going to have to come off the fuse box (area) and run a wire to the trunk for a B+ powered light… And you could probably have it done in the time it will take you to find, get and read a wiring diagram… lol
EDIT: Trunk release if it has one… (I don’t remember)
Just get the helm schematic, and lose the LED lamp.
Why would I lose the better brighter LED bulb? 
This forum and others get many posts where LED lights in vehicles that did not come with LED have electrical problems .
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LED lights don’t provide enough resistance for turn signals to operate. Many late model vehicles monitor lamp circuits for fault detection, LED lights can cause a malfunction when used in place of an incandescent bulb. I don’t think these problems apply to your Pontiac’s trunk light.
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Those are most likely CAN bus systems.
Or someone using CAN bus bulbs in a non CAN bus system.