I just went to a BMW dealer and bought a tube of BMW "Convertible Rear-Window Cleaner" (the box says "Cabrio Heckscheiben-Reiniger", part # 81 22 9 407 665). Cost me $8.50 - I called in advance to ensure it was in stock. I wasn't able to find it online, but now I see that searching for "Cabrio Heckscheiben-Reiniger" yields some hits online.

I had a pretty gunked-up rear plastic window for which Plexxus was adequately helpful. This BMW cleaner took the gunk off much quicker and with less effort than Plexxus. It comes in a toothpaste-type tube with essentially the same consistency as toothpaste. Wipes on and off pretty easily, taking with it all of the gunk that was still on the window. In general, I'm happy with the results.

HOWEVER: Be very careful in applying this stuff, as it is very easy to get it on your top, even if it's just the edges around the window. And, it turns bright white, which does not look good on your black top. The white will come off, but it takes some effort and it can be very hard to get it out of the crevices (even harder to get it off on the inside). I was too casual in applying it and it actually took more work getting the white stuff off of my black top than it did in getting the gunk off the window.