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Most likely this is for the seatbelt buzzer. The bulb for the fasten belts light is just a socket that twists in like the rest of the bulbs in the cluster. The buzzer is controlled by the seatbelt. If the key is turned to "on", the buzzer will sound for about 5 seconds, or until the belt is pulled out. It isn't connected to the buckle, as I assumed it would be, but to the roll of the belt. Being as though you have aftermarket belts installed, I doubt there is anywhere to plug this in for the buzzer to work.As for the dome light, my dad's 80 Custom Deluxe does not have a headliner but does have a dome light. I can't be certain, but I don't recall any wiring running to it, so it must be hidden somewhere inside the metal panels. I can't be certain though - haven't even been inside his truck in years - but I can check for you.Jim
The dome light wire runs through the roof panels and down the rear drivers side post. The connection to the harness is in the rear corner. Sent from my SAMSUNG-SM-G920A using Tapatalk
Just an additional option or two for your interior lighting. I noticed that under the dash on the passenger side there was a place where a light socket could be mounted, so I found one at the junk yard and tapped into the wiring for the other under-dash light, and this made a nice addition at night. Also, I have an '86 Olds with T-tops that did not have a dome light. Someone told me the wiring to the interior opera lamps still ran behind the trim on the middle bar in the roof, and I could tap into that. So what I did was found a mirror from a late 90's, early 00's car that had integral switched reading lamps (I believe it came out of a grand prix or lumina or something of that nature). If you are able to hide some thin wiring along the top edge of the windshield/roof and then behind the trim on the A-pillar, this might be a viable option to get you an overhead light. If you want more info, let me know and I can help describe this better.