I like a stock looking car myself but with hidden upgrades. That said I have yet to find a decent solution for a better sounding system in a convertible that I like.

I had a AM Radio converted by Turn Switch for my 64 Tempest Convertible and will most likely do the same for my Bird.

Most go with the kick panel speaker but I like my vents. A good set of kick panel speakers do sound good.
Sound quality will only be OK using an under dash speaker. I have used both a dual Kenwood and a dual coil style single cone speaker.
The factory rear speaker(s) have been covered and as stated are worthless when the top is down.
Most other options involve cutting metal Doors or side panel in the back seat area
The only other option is a floor speaker setup like trucks use. I tried to see if Undercover II speaker would fit under the seat and they do not.

I did see at a car show where one person had drive in movie theater speakers hanging on the inside of the car. I thought that was kind of cool.