Tail Light Nut Help - thread size / pitch. 91 Caprice

Hello All,

Either the previous owner or I lost one of the plastic wing nuts for holding the tail lights on, and another is stripped so bad it's loose. This is causing water to leak into the trunk through those pass-throughs. I wasn't able to find these online anywhere, ordered a set from ebay that claimed universal and included caprice in the title, but I think this may have been for previous gen as the hole was much larger and slid right over the threads. Took a thread gauge to the bolts and came up with 10-32, I then thought I confirmed this by taking the one of the wingnuts into Lowes and it threaded nicely onto the 10-32 post on their thread gauge.

Found a 10-32 nut (no wing nuts in that size at lowes) and some washers however the nut still wouldn't fasten. Thinking a heave buildup of junk on the threads might be an issue, I used a die to re thread the post. This ultimately worked for the one missing and got it secured at least although it doesn't look great.

Does anyone know where I went wrong here, or know the exact thread needed for these? Is there something special about these? I'd especially appreciate if anyone knows where some of the original plastic ones can be obtained, but from searching around here it seems a junkyard may be only option.

Car is 91 Caprice Classic, thanks!