Rear do not drain at all. I could easily suck the water out at the end (hose end is behind the tail light), so the line was clear but water refused to drain by gravity. The problem is poor design of the drain hose routing. It bends upwards before going down to the tail lights. Water has to flow upwards which creates a plug for air in the hose and the gravity force of the small amount of water in the drain corners is not enough to push the water through the hose. In addition the hose was pushed below the wiring harness to the lowest point and then goes upwards because of the bend. An easy fix is to cut the hoses at the bends and install 3/8" elbows as well as raise the hose above the wiring harness. This creates a constant downward slope for the water and prevents that water is trapped in the hose. Now the rear sunroof drains work like they should (and like the ones in front).

Original design, upward bend traps water and plugs the line (rear side panel removed):


3/8 ****** elbow, local hardware store:


Elbow installed (requires some silicon grease and hot air gun):


Hose pushed down from above and pulled down from below at the same time (remove tail light):

 

Good solve. Fun fact: In Solihull, the water flows uphill. So it makes sense why they did it this way.