I also had a similar issue recently with finding the start of the tubes where the roof sits… After about 15 minutes of poking around I gave up.
If you go under the car and look on the inside of the tub for the outlet as RichardFX said above, you’ll be able to feed your pipe cleaner from bottom to top instead and therefore it will reveal the exact place that the cleaner needs to go in from at the top of the car… Makes life way easier.
Once you’ve cleaned them, pour a decent amount of water into the drain channels to flush the remaining bits of muck out… I was surprised at how much more mud came out after I’d already cleaned them with a trombone cleaner