Although I agree totally with Orionson, I also appreciate that we are entering the car from the other side. North American drivers will be sweeping then swivelling against the flow of the pleats. Also many fabric carseats don't allow for a swivel as they seem to grip cloth.

There is an other option. Obtain a new plastic carrier bag. Cut down both sides, to give you a large rectangle with a handle at both ends. Sweep your pleats using the bag. Depending on the size of the bag and the size of your aft end, you might be able to transfer both handles to one hand. (This is the ideal as it gives you a spare hand for balancing, adjusting or what ever as you sit.) Sit and swivel. Ease the bag out from underneath yourself, pulling in the direction of the pleats. A final wiggle for comfort and you are done. Stow the bag in side pocket of door or glove box, for the next time.

If it works for you, then it would be advisable to look round for a heavy-duty piece of plastic - preferably in manly black. Garden waste bags are usually made of heavy duty plastic.