I went to a music store and bought a trombone cleaning brush which is a flexible cable about 3 feet long with a brush at either end. You simply (with a bit of practice) feed it through the opening in the back panel and it forces the accumulated build up of leaf litter out the waste plug under the car. Best to start by getting a box or similar to hold the roof partly open to give you access (and yes, I know my car is putrid but it's Saturday morning here and it rained last night :-( Then, take your brush: Find this hole And with a little bit of trial and error you will find the correct angle to feed the brush down through the drain hole If you find it hard to get the brush in the hole, it's quite possible that this is because there is a large amount of litter at the opening of the hole so just be persistent and you'll find it This shot shows the angle of entry Once you've fed it through, make sure that you reach under the car (sorry, couldn't get my camera under the car, she's too low) and pull the brush out of the hole from under the car as this will pull any additional litter that the second brush traps. If you're doing this for the first time, position a container or similar under the car to catch the crap that comes out, you will be very surprised !!!