The silver posts are used to actually hold the dome light assembly to the roof / t-top T bar. Slide the plastic dome housing over the two posts, then use circular clips to hold the dome light and headliner up in the center of the roof.
Remember that the black part of the dome light, behind the white lens cover actually faces the front of the car. At first it will look like the dome is facing backwards, but thats the correct placement.
Be happy its just the dome light. If you were hanging an overhead console, hanging a new headliner becomes a lot more tricky.
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I USED TO HAVE an 85 Camaro 305 4bbl, 90 Firebird V6, 97 Firebird V6 (10 speakers!). Despite the two family vehicles, 3 kids, wife and mortgage, I can finally realize the dream - 88 TA almost all stock... for now.
http://www.cardomain.com/id/untameddave
Wanted this -

, but ended up with this -

for free. Can't argue with free.