I mounted the sub to the V shaped frame behind the back seat of my Cuda. I had room to spare. I had to bend the spare tire mounting rod in order to move the spare toward the rear about 3 inches to make room for it. I bolted two Alpine shallow mount 4 ohm, 4 inch speakers together and installed them in the dash with heavy duty velcro from Home Depot. I paired those with two more 4 ohm 5 1/4 inch shallow mount speakers in the kick panels. I don't think that you can mount regular voice coil speakers in the trunk and expect to hear them very well. I'm not sure where you can mount the rears in a convertible.
Here is a link to the sub:
http://www.pioneerelectronics.com/PUSA/Car/Subwoofers/Subwoofers-By-Size/TS-SWX251