Other than the doors, the most common place is from the sunroof and/or drains. Since you don't have one, it isn't an issue.
Check the overall condition of your door seals. Make sure you don't have any tear, rips or other problems. Ensure when your door closes that it seems to fit correctly. If the door had ever been adjusted, you may have a gap somewhere. Another good preventative measure is to regularly treat your door seals with something like Meguiar'* Vinyl and Rubber Cleaner. This will also condition them. There is also a silicone emulsion available through your GM dealer.
The doors seals don't need to be "adhered" to the body with silicone or other sealants. These are meant to be removable and I have never seen anywhere in the FSM that indicates they need to be resealed with any sort of compound. I've had mine off several times without additional problems. Just make sure that they are applied evenly without any large gaps between the seal and the lip on the body.
It is also unlikely the water is coming from your firewall unless someone had previously pulled out the wiring harness passthrough(*) and didn't seat them correctly.
My best guess (until checked and found to be fine) is still the water deflectors inside your doors. If you don't see anything obvious with the bead of adhesive, close the door and really soak the doors down well. I've seen instances where everything "looked" okay, but with a water test found the leak.
StingRay Bike w/cards in the spokes, Weeeeeee!