Black RTV under the antenna gasket on the roof. You have to get the nut loosened way up on the threaded nut.
Now if you have someone push up from in the cabin, so you can lay a pretty bead under it you will be crackin, but inject some in the hole first.
I managed by myself somehow. That was 8 years ago and hasn't leaked since.
I probably used silicone spray (like removes like) to get rid of the excess. Be careful not to mar the area.
One pass section swipes with a microfiber saturated with silicone spray and another to wipe it dry.
This link from GPONA has the dome light disassembly from the GM manual towards the bottom of the page.
http://www.gpona.com/forums/viewtopic.php?t=2033&highlight=star+leak
edit.....probably leaking sauce because of that damn bra dealie LOL