When I bought my van several years ago, I was getting water leaking in the drivers side behind the dash, soaking the mat under the carpet.  I tried to seal the windshield several different times all no good.  It still leaked.

A few weeks ago I finally got around to putting sealant on the antenna gasket and over the entire base of the antenna- nut,  in an attempt to stop the leak. Well the leak stopped and I have been through at least five heavy downpours and still NO LEAKS.

So if you are having leakage, seal the gasket on the base, also put some on the outside of the antenna to prevent water from getting in through the top.

Robert