That's a tough one, as there is hardly any room to put them.
The previous owner of my car had installed them in the rear side panels which, of course, he had to cut up for that.
Problem here is, besides cutting up the panels and ruining them for good, is that the speakers have no closed chamber around them, so they do not produce bass or volume and sound pretty thin, especially when the top is down.
The back of the speaker needs some resistance from an enclosure so that it is able to move air. With the top down, the panels are wide open at the top and the speaker has no power at all.
I found some original panels that I will probably put in my car, so I'll put the speakers in the doors.
For some "ooomph" you might add some bigger speakers in the trunk that face the back of the rear seat. They will not produce HIFI sound, due to the vinyl of the seat, but should be able to produce some "torque" in the low frequencies.
But I am no expert when it comes to car stereo systems, so somebody else might be more knowledgeable.
Anyway, here is a pic of my speakers in the panels: