DIY Drain Cleaning & other findings...
I also have a 2006 530xiT that has the sunroof leak. Mine was the right rear.
Cleaning the rear drains is an easy DIY, even if you do not remove the headliner. The rear drain tubes run behind the fuse box on the right, in the luggage compartment. There is a small access hole behind the fuse box that you can just barely get your hand into (& I have big hands). It is similarly located on the left. Once you find the drain tube you can either pull it out 1st (it is fastened in place with a rubber "leash" & has a flapper valve on the end that has to fall off, so it is hard to pull out) or go under the car, remove the under cladding, find the tube (it is hard to find) and pull of the flapper valve then. The flapper valves just restrict junk getting out and leads to clogging- at least it did in my xiT. Either way (just pull from inside, or take flapper valve off 1st from below) you can now pull drain tube out of the access hole on the inside of the car. It was clogged. I cleaned out the junk at my end, and then blew into the tube and felt the tube pop when more blockages cleared. I tucked the tube back into the access hole, and through the 1" hole that leads to the bottom of the car, and put the underbody cladding back on.
If the drains work properly, they will drain when water is poured IN THE OUTER CHANNEL ONLY with the sunroof open (as lokking down on an open sunroof). DO NOT pour water into the inner channel as this bypasses the drains and causes water to drain directly into the car, simulating a leak EVEN WHEN THE DRAINS WORK AS SUPPOSED TO.
I believe the design works this way:
The design is that only the outer channels are supposed to direct water. What happens is when the drains clog, the outer channel fills and overflows into the inner channel. The inner channel then drains into the inside of the roof, down the back and into the electronics! When the drains work properly, when the roof is closed, and if the sunroof seals leak, the water falls into the outer channels and drains through the 4 corner drains.
hope this helps.