How effective would a tool-well mounted 10w3v2 be in our car? I've had no prior experience with roadsters... and I don't know if this is a universal thing or not but the Z3 Roadster's trunk is completely sealed off from the cabin.

 

You would be good to go. While it is not the ideal car to build a stereo in it will still sound good.

Make sure you dynamat the trunk and plug the trunk vent hole