Sorry, this post got out of hand, but if you read it it should fill you in on the problem:
If you're getting a river of water coming through, go back and review your sunroof gasket sealing. Maybe it's not positioned correctly and one corner isn't sealing, or perhaps a section became damaged during the install process. The overall strategy should be to get the glass flush ...if not even a hair higher than the roof, so water will shed off the glass/roof, not pool on the corners. It took me a couple readjustments to get the seal right after installing the new gasket. Also, make sure there isn't any dirt, debris, or cracked/flaking paint on the perimeter where the gasket meets the steel roof. All of those things are possible. The gasket should keep the water out. The drains are for secondary, safety whenever water may intrude. But they aren't the primary means of water management. What commonly happens is the drains get plugged up with dirt/bugs/debris and water that gets *past* the seal backs up and sloshes on yer lap when you take off from the car wash (or driveway, etc).
As for the drains, the little grommets on the ends are the location of the typical cloggs. They are located high up under the dash on each side about where the upper door hinges are. On the driver's side you can see them with a flashlight. Looks like a small black hose about 3/8 or 7/16" diameter. They have special bends and are semi-rigid. You can pull the grommets out of the firewall like pulling out a regular rubber grommet. Then it's possible to pull the grommet off from the end of the tube and clean it all out. There are fingers inside the grommet which keep critters from crawling in and yeah, those fingers catch crap draining down too, and cause a dam. It's a small opening fore sure, a little fly could get in there and plug them up. Clean up the grommet and tubes and replace everything. The pass side is accessed by removing the edge of the dash on the pass side (you literally ..but carefully, pull it off. There are reusable clips). It's a tough reach but I was able to do mine.
Another issue is the tubes go horizontal about where they go to the firewall. Then the water velocity slows down and any sediment drops right before the grommet fingers (a double-whammy). The overall key IMO is to clean and keep clean ...and periodically run some water through the drains to flush out debris and sediment.
Back to the cleaning process, after I removed my firewall drain fittings (those molded rubber hose ends), I found using a thin 1/16" cable about 8' long (from the hardware store) for like $2, and taped a small piece of plastic film (from a plastic bag) somewhere in the middle of the cable. Then fed the cable down the drain tube from the front corner of the sunroof assembly. Then I poured some warm soapy water down the drain and worked the cable (and plastic wad) up/down a few times on the end to clean the inside of the tube. (note: I hooked a short section of clear plastic hose onto the bottom of the Hummer's sunroof drain tube to a bucket inside the cabin to catch the water). Same thing for the other side. Be careful on those semi-rigid tubes you don't get rough and crack one if you're working in the cold. They would be a bugger to replace.
Then I completely cleaned all the crud out of the sunroof assembly tray at the top, including the sides. There was a good number of dead bugs, dirt and debris all around. Then a final flush of water to ensure everything was nice and clean up there. I did all this on a nice sunny summer day. Afterwards, it's a good idea to not to park under pine trees or leave the sunroof open unless out in the open. In the fall, I poured a cup or two of hot water down the drains to flush them clean again ...which only takes a couple minutes, a lot easier than going through all the work of removing the tubes and cleaning the grommets. I'll continue to flush them periodically. But since getting my gasket settled I haven't had any leaks through the gasket ...or even wetness to the touch, even in hard rains.
Most guys say to cut those fingers protruding inside the grommet. I personally think that's a bad idea. There's an engineering reason they're there. One reason is likely to prevent cold air (and maybe even CO from an engine leak) from intruding back into the cabin. Another is likely to keep insects from nesting inside the tubes. Many other vehicle manufacturers that have drains use some sort of anti-insect intrusion/prevention measures, not unlike the fingers you see in that hole. Some mfgrs that have weak anti-insect measures I've had bugs build a mud-like nest in and then you get back-ups too. But whatever you want to do. One of the main oversights from the videos people make on youtube is thinking the drains are the problem for the leak. The root issue is usually a leaky sunroof gasket in the first place and an abundance of crap in and around the sunroof frame. Bugs, moths, small leaves, pine needles, mud ..they're all going to be a problem so best to make sure it's kept clean up there. And the drains clean too.
There are 4 drains. The rear drains as far as I recall don't have those fingers inside them. Probably because engine compartments can be high-pressure zones in vehicles and could possibly 'pump' bad air or cold air into the cabin if too much of an opening was created. Just my thought.
