Hellooooo - Did some mention Cab ICE??
I have a set of 17cm Kenwoods in the rear shelf, and they do sound great with the roof up, but extremely muffled with it dropped. I did have a remote switch in the centre console to kill the Amp that supplied the power, for when the roof is down. But now I just leave them on, and it doesnt give any problems. It is worth upgrading the side pocket speakers in the rear to a qood quality set and running them from an amp. I have done this too, and this combined with the front ones driven from another amp, is quite adequate to enable the stereo to still be heard with the roof down, at motorway speeds.