Sounds very like the sunroof drains to me, is this on your d3?
I did mine back in the summer but could only do the fronts as could not see the rears and to get access to them you need to strip the rear wheelwell arch liners out.
To clear my front drains I got a killa spray bottle from b & q.
If you take the lance off the hose and cut about 3 inches of the hose of the end, and assembly the lance back on, now take the piece of hose and replace the spray nozzle with the bit of hose (just push it over the end of the hard pipe)
You now have a bit of hose that fits snugly in the drain hole of the sunroof and you can pressurize the bottle with air first just incase the drain pipe is split and not blocked and then try to clear the blockage both sides.
When i did mine you can clearly hear it spit the crud out the end down near the front wheels.
Then as a belts and braces i filled the bottle with tepid sopay water and pumped a few litres through each side to flush any remaining debris out.
Good luck
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