Mr. Volvo said it better than i could.
@realrobd Are you sure the trunk light is working as it should? Why would it be designed to come on only when the interior lights are on? That doesnât make sense to me.
Personally, If this is what I was after, Iâd run a new wire from the fuse box to a mercury light switch in the truck- or even a manual switch so I could make sure it was off when it was supposed to be off. Fuse the wire and enjoy.
1 Like
Thinking about this, it makes no sense at all. Everything goes back to the bcm and controlled by one fuse. So you have to turn the dome light on or have the doors open for the trunk light to work?the library should have a manual with the diagram but as said unless it is helms, canât rely on it.
The first thing youâll need to do is locate the wiring diagram for your vehicle. You can find this in the ownerâs manual or you can search online for a copy. Once you have the wiring diagram, youâll be able to identify the switch that controls the trunk light.
Once youâve identified the switch, youâll need to trace the wiring from the switch to the trunk light. This will help you determine how the switch is wired and how it is connected to the trunk light. Once you have this information, youâll be able to rewire the switch so that the trunk light will illuminate when the trunk is opened or closed.
If youâre not comfortable with rewiring the switch, you can always take your vehicle to a qualified mechanic or auto electrician. They will be able to help you rewire the switch and get your trunk light working properly.
I hope this information helps you out. Good luck with your project!
The trunk switch provides the ground for the trunk light, no need to rewire the switch. The body computer supplies the power to the trunk lamp.
The only factory service info I see there for a 99 Grand Prix is an emissions system diagnostic manual. $50. Do you think it would have the wiring diagram the OP needs?
The OP wants the trunk light to turn on whenever they open the trunk, which would require BCM to supply power to that switch circuit at all times.
Unlikely to be in an ownerâs manual. The needed diagram is probably posted on-line somewhere, but may take a good deal of searching to find it.
Even a factory service manual can contain incorrect information.
Thatâs very rare.
But very common with Haynes/Chiltonâs.
Tester
1 Like
The trunk switch is like a door ajar switch, it only has a ground connection, no place for a power feed.
Basic diagram below;
I guess other pontiacs may be different but I have a hot power wire going to mine. Ground is on the metal body to complete the circuit. Made me look in my trunk but Iâm done with this one now. Iâm just not convinced of the original symptoms.
Iâm late for my last class on the federalist paper though so gotta go. Everyone is free to do what they want
1 Like
Iâm unsure of your point. The diagram youâve shown is exactly the functionality the OP wants, trunk light turns on whenever trunk opened. Are you saying b/c the + voltage isnât sourced directly by the battery in OPâs car , but from the BCM, the OP cannot obtain the desired functionality?
No dispute, agrees with my experience as well, especially for wiring info. But for something like a trunk light wiring diagram, what would you recommend if not Haynes/Chilton? OP attempt to locate & purchase the factory service manual? OP attempt to locate the diagram on-line? OP hire a shop to rewire the light the way they want it?
You do not apply power to the trunk switch, that switch is a ground only.
Tester
Ok , I should have said âcircuitâ rather than âswitchâ. Will correct above. Is it possible for OP to obtain the functionality they want?
I think anyone can wire up a lamp as long as they donât create a short to ground.
10 minutes after exiting the vehicle the BCM disables all courtesy lights to prevent a battery discharge, that is why there is no power to the trunk lamp when returning to the vehicle. Should the trunk ajar switch wake up the BCM? I donât know, I would just use the keyless entry remote to unlock the doors and thereby switching on the courtesy lights.
Re: Mitchellâs online manuals
My only Mitchellâs repair manual experience is with their prior big books. Many years ago I installed new clutches on my truckâs rear limited-slip differential using a Ford 9-incher Mitchell chapter for reference, as I recall. I found it to be generally pretty good, comprehensive info, but like All-Data, geared to mechanics. They assumed the reader had quite a bit of experience doing car repairs and car-repair nomenclature.
Besides Mitchells I seem to recall another vendor who published similar auto-repair info in big books, maybe started with the letter âCâ, but forget its name.
The Mitchellâs online price seems pretty reasonable. $20 for a month, $40 for an entire year, $60, four years. imo OP should definitely consider this option to obtain the needed wiring diagrams.
OP, is there a reason this method wouldnât accomplish what you want?
Lights do not come on when unlocking with the fob. Only when opening the door.
It appears @Nevada_545 's suggestion wonât work. Back to the drawing board I guess.
OP, have you decided yet how youâll obtain the pertinent wiring diagram? Or are you just going to wing it; i.e. use whatever diagrams you can find on Google, etc? Testerâs post above that Chiltonâs Haynes wiring diagrams can be inaccurate is true from my experience, but the primary reason imo is that those manuals cover a bevy of models and model-years. So while a particular wiring diagram they show for a function (like the trunk light) may well be an exact copy from the manufacturerâs factory service manual, it may not be the exact diagram for your particular model/year/options. You are in the best position to decide for yourself how sensitive you are to wiring diagram imperfections.
Another idea, suggest to stop by your local public library. They may have the applicable Haynes or Chiltonâs manuals you can reference there. Also ask the staff if they have any other auto-repair reference materials, such as on-line reference sources.
Illuminated entry is a standard feature, you must be experiencing a malfunction.
1 Like
Looking at the manual, I can confirm this should be the case.
The plot thickens. 
Do any interior lights at all turn on after unlocking a door using the remote? I presume you can unlock the doors that way, right?