Leaking front windshield or OnStar antenna?

It may seem a bit primitive, but I would use the brute force method and take the garden hose to it. Force a stream of water at the antenna housing to see if you get the leak. It very well could be a bad seal. I wouldn't think it is the windshield seeing that it is leaking above the level of the top seal of the windshield.

xwrightxjax did you happen to find out why your lacrosse was leaking my aunt a 2018 Buick Lacrosse and hers started leaking like this.

Hi- I had the same problem with water ingress. After clearing all the drains in and around the wiper area, I discovered that the leak was from a body seam that had lost its seal. Removing the front wheel cover allowed me to find and reseal the leak. I found Flex Seal filled the gaps well. While the leak seemed to be on the driver side, I took no chances and sealed both sides. The leak was at the first top bend and again about halfway down. See photo...