Well... I pulled most of the headliner down today... Enough to gain access to the rear sun roof drainage system. This includes removing
1) Both rear passenger door carpet plastic trim pieces (easy to remove)
2) Both C pillar side plastic panels (fairly easy)
3) Panel above midgate window (this one requires a ton of force to get the fir-tree style plastic anchors out)
4) Both headliner fir-tree style plastic anchors (easily done with 2 flathead screwdrivers)
5) Both B pillar side plastic panels ( The top clip was TOUGH to get out on one side... I ended up damaging one but I think it can be bent back, and another one pulled out but left its metal clip in the trucks sheet metal... I think it can be re-attached)
6) Removed both rear "oh-@#$%!" handles. Easily done with a 10mm socket
At this point you can pull down enough of the headliner to get to the drain system. There are two velcro stickers that will also easily come apart when you pull it down a little. I did not remove the sunroof trim, but it might be a good idea. I ended up having to push part of the headliner back in to the sunroof trim when re-assembling.
On my 2003 I discovered there are not just 2 rear drain holes, but 4! This makes a total of 6 drain holes (2 in the front, 2 near the rear, and 2 in the rear, of the gutter system). Both 2 rear drains on each side are joined together with a Y hose adapter and empty down the B Pillars.
The hoses easily twist off... And, you really don't even need an air compressor to blow them out. You can get your mouth on them and blow... I used an air compressor anyway after doing this... but I don't think it was necessary...
To my surprise the hoses were not clogged... I did however pull out a leaf, and a maple tree propeller from the rear of the gutter that could have been blocking *some* of my flow.
I tested the system by opening the sunroof and pouring water in to the gutters. If you pour too much at once, I believe even any Avy won't be able to handle that much water and leak.
There is an flaw in the design of this gutter system. If you park your car on even a *slight* grade incline (engine above the rear of vehicle), the water will have to run UPHILL to make it from the rear drain hole to the top of the B-Pillar. This is not possible so the rear gutter overflows, goes over the plastic dam that leads into the next chamber, which has a hole in it (because this is where the plastic fir-tree anchor goes), and guess what? Headliner gets wet. So, if you leave your fir-tree anchors out, the leaking water should pretty much come straight down without soaking much of your headliner. I suppose you could even go ghetto and attach a drain tube here if you expect heavy rain.
I really wish they could sacrificed a little bit of rear passenger headroom and ran the rear drain hoses down the C-Pillar. Without a redesigned headliner, I don't think this can be fixed.
Anyway... I took a few pictures to share along the way...