Check under the wiper cowl cover (you have to remove the wipers). On the right side (pass), there is a fresh air block off plate that gm did a horrible job of adhering. It will need to be removed. and re-sealed - maybe with silicone. The other options would be if the sunroof drains are clogged, and finally is to check the bottom of the fender for leaf accumulation. It can be accessed when the door is open and you remove the small rubbery block of piece that is kinds shaped like a triangle. Once removed the leaves can be hosed out. This causes rain and road water to enter through a higher than normal portion of the door seal, sometimes through wiring/door switch grommets. I pretty much had to do these to fix my leak issues. Of course if it smells like coolant then it is a heater core leak, but not common on these.
