I discovered that what appears to be a gasket under the antenna base on the roof is disintegrating...has anyone replaced this? It appears to be a part of the antenna base as a whole and is attached with a nut securing the coaxial cable in the recesses of the headliner...I hope I am mistaken.

 

I don’t think you are mistaken. I believe the headliner has to come out to access the antenna base.

 

I seem to recall a thread regarding dropping the headliner, but I can't find after multiple searches. Has anyone done this? Is there a DIY? The rubber gasket on my antenna base has split and I don't want water getting into the car through the now more exposed aperture.