Each side of the PRHT has a tubular drain from the rubber seal around the base of the back window, and a moulded plastic drain from the top of the B-pillar near the seat-belt guide. These discharge onto the grating which then exits through the flaps to under the car.
Looking up at the back window drain tube connection
I’ve not tried to remove the gratings yet, a good strong wet-vac underneath the car held over the outlet with a hand making the seal better seems to be highly effective - no brushes needed.
Hot soapy water seems to free off most dried-on gritty deposits and helps the flaps seal better afterwards.