It is hard to determine if it is leaking from the side gasket that seals the two windows side by side or the bottom rubber gasket on the slider. My experience in the glass busisness for 25 yrs would be to start with drying it out the best you can with towels. Then have someone sit in driver seat, and you get a ladder and a garden hose on low, and start at the bottom. Work your way across and up slowly. This will narrow down where the leak is exactly. It really could be coming from up higher and traveling.
Let me know what the results are & take it from there. It may, but hoping not, involve pulling out the whole window.