This is an old thread, but it's getting to that time of year!
I use the autoglym kit for convertibles. Sounds similar to the BMW kit in that you get a cleaner and waterproofer. Enough for a couple of applications. Once applied the water beads up just like on your lovely waxed bodywork.
Speaking of which I've recently started using their aquawax as well. Only takes a few minutes to apply after washing, but really helps you keep on top of the cleaning.
( starting to sound like an advert, but I don't work for them honest! )
