What a PIA job. Just as 2005JGC described, the rear drains run down the C pillar (inside) and down a narrow gap between the leading edge of the rear wheel fender between the inner and outer fender. And yes, they are encased in expanding foam. There is no way to get to the end point and the foam glues in the lower half of the drain tube.
Since my headliner was tough to get enough of a gap to get my hands in there to reach the drain tube at the sunroof, I chose instead to remove the rear quarter trim panel. This is fairly easily done, 2 screws and a few edge clips. Start by removing the door sill plate and work up the wheel arch side, removing the upper C panel and then unclip the area in the cargo bay, where you will see 2 Phillips screws that also must be removed. If you have a 12v outlet in the rear, once you move the panel slightly out of the way that can be disconnected, and remove the side panel altogether. With the area exposed, remove the seat belt retractor, 1x T50 bolt and then peel away the rubber baffling. With the hole exposed, you can now see the rear drain line. I chose to cut it at this location so that I could blow it out both upwards to the sunroof and downwards to the drain hole. Luckily, there were no physical obstructions. Someone mentioned that it sounds like a Moose call, and sure enough it does, regardless of which end the air is blown thru. I had silicone tubing that was a tight fit to slip over the drain tubing to recouple the two halves together. Also leaves you a service port for future maintenance as needed.
I'm pretty sure most folks who tackle sunroof draining issues only do the front pair and may leave the rears clogged. All sunroofs that I have dealt with over the years have 4 drains, 1 on each corner, but the rears almost always go down the c-C pillar, and are nearly impossible to see from inside the sunroof assembly with the sunroof open. Luckily in MOST cars, unlike Jeep, the drain end point is accessible under the chassis. Why the chose the route they did vs running the drain port after the rear wheel arch, as mentioned in the fix, is beyond me.
Thanks to 2005JGC for all your help, very informative, and my drains are now....hopefully, ready for the torrential rain storms we get in South FL.